Vaneras is right in most of what he posted.
The primary means of acquiring new gear is through defeating content like for an example raids, whereas Valor Points is a supplemental means of getting new gear.
That sums it up pretty nicely. You can debate the valor item ilvl in regards to raid item equivalents of course, but in essence, that is true. Valor items are supposed to be 'in case you are unlucky'-gear, and if you are lucky, then you now can choose to upgrade instead.
So why the cap? As said, it enforces a choice, and provides an alternative way of gearing in a controlled steady pace.
Trust me, Nothing
(!) good can come out of a no cap on valors. E.g. With the introduction of upgrades, high end raiders will be 'forced'
by their raidleader to do daily hc 24 hours a day (40 valors a go). Fun times. If you dont, you are not doing your best to have all your gear max upgraded, and not doing your best in terms of progress = worst case kicked out from the raiding team. If there was no cap, every raider and everyone hoping to be one, would rush through 3000-4000 valors a day until there were no more valor gear to be had, and no more upgrades possible. What if you have to choose.. a 2 week vacation on a sunny beach with a beauty queen and kicked out of the team, or 53760 (+560) valors (which is 2 weeks of doing 4 hcs an hour nonstop.) ? And in all honesty, blizzard should not encourage to not live your normal life. In fact they should do the opposite. The old text based Utopia game (if anyone remembers it) had one nice thing in this regard, that if you didnt log in at all for 20 hours, you got an offline gold bonus. That is something blizz should consider and implement somehow. But that is going offtopic.
While being slightly offtopic, I must add to this discussion, the essense of the implementation of valor items in the first place, and I will focus on the second part of the quote above.
Valor Points is a supplemental means of getting new gear.
As such, valor rewards should NEVER
(!!) be hidden away behind reputation requirements. That is so illogical, as it forfeits some of the purpose, in response to Vaneras statement there. Valor items werent behind reputations when valor were introduced, and I cant really see a valid reason why they changed that. E.g. I now have around 7000 valors stored in gear on my alt priest, not being able to purchase for the slots Ive been unlucky for (the purpuse of valor gear), just because they are hidden away behind a reputation barrier. And yes, I hate doing dailies, especially when you have 10 90s, working on your 11th. Dailies are the thing in wow furthest away from being social as Greg stated would want us to be. You hate if another person is close by when you do a daily, stealing your mob/quest items. And if you repeatable say "Dailies arent needed", then please adhere to the essence of the original valor item implementation and just have them as a supplemental mean of getting new gear. Without reputations. The reputations can do other things, such as buyable gear, achievements, mounts and/or unlock quest chains/portals and other stuff.
But returning to the cap itself, the current model is unfortunate in many ways. You have caps on how fast you can acquire valors/reps/golden charms etc, but they are absolute on that day/week. With that I mean, that if you miss a daily, a week of raiding etc (like christmas week for many), you are irrevertably one week behind. If you have an alt leveling up now or new to the game, you will be 3 months behind. If you miss a few valors for your legendary, then you miss a full week, Which brings us to the following commonly heard statements :
MoP is very alt unfriendly
We are trying to find ways of people catching up without negating the value of achievements from people there from the start
Which brings me to my 'magical suggestion' :
Let the cap be rolling every day, and not on weekly basis. Let me give you an example.. When the tier starts, valors are reset as normal. On day 1 you can earn 150 valors (all sources), on e.g. day 49 you can earn a total this tier of 49 * 150 = 7350 valors. This will actually be virtually no change at all to top raiders, as 7 capped weeks is 7000, actually a slight improvement for them. But more importantly, if you miss a week, you can easily catch up. You can go away to that beach, and still feel you didnt let your teammates down when you come back. You can catch up. Or you as a new char can catch up on your friends, doing a bunch of hcs, without getting anything for free, negating the efforts they did. Since you still can only have 3000 valors at any given time, you are still left with the same choices on what to buy on the same tier gear, at week 7, as others had week 1-6. I cant see anything negative about this change, as this will also help push people away from rushing everything at wednesdays. And if you raid on wednesdays, you will cap that day, regardless what the cap is that day.
This more alt/new-friendly model should go for valors, golden charms (1 every 3d day or something for 30 lesser), yes virtually anything that has weekly/daily cap AND is tied to a tier.
And related to this, you have the different bonuses they have implemented for valors and reputation. They are good and help alts, but unfortunately the reputation commission bonus (100%) and the valor cap bonus within a week (50%) both act the same unfortunate way. Right now, the most benefitial way is to cap one character as quickly as humanly possible. That is, do dailies every day until revered on one character before doing anything on your alts. And cap 1000 valors on one character on wednesday morning, leaving that character for the rest of that wow-week. This really isnt supporting diversive play, and is actually contributing to other negative effects as well. e.g. it feels wasted to use my scrub tank alt in an hc, before I have capped one of my chars, even though the satchel is there and the need in general is great.
A simple solution for this unwanted 'burst play', would be (perhaps a gold paid (?)) choice to instead of having the current 50%/100% model on valors and reputation, you could instead REDUCE
the valor gains you gain on your current char with 20%, but instead gain the same amount on ALL ELIGABLE
chars. E.g. I choose to use this, and instead of the 5 valors on a daily, I get 4 on the current char and all other 90 chars I have will get 1. I then switch to an alt and do the same daily, he gets 4 and all other chars get 1. After those 2 dailes, 2 chars have 5 valors, and the rest have 2 valors. (2 chars will also get e.g. 150 rep, while the rest will get 30 rep). Since all chars are capped by the same tier wise valor cap (as suggested above), and you have your own choice of activating it, noone will be forced to use alts to benefit a main. As if you use an alt when you have capped a main, you'll only get 1 justice point on your main.
This will have other positive sideeffects, beyond addressing the downside above, as it also will provide a benefit for people using their characters beyond cap. As this will (very slowly) gear up your alts as well. And even bursting your main to extreme wont matter much, as at day 49, your alt is limited by 7350 valor cap anyway and would have gained it faster gaining it himself. But it will never feel wasted to swap character at any point when you play.
Once activated this will affect all your chars on the same realm. And this will remove the need for commissions and valor cap buffs (as a related side note, in the current model, a much more clear visual is needed in your reputation pane about the commission buff is active or not).
Sorry for multiple angles and the wall of text, but It kind of addressed most of the concerns of what people have posted here and related in other threads about gaining valor points in general. Its all connected.