There's merit in every model described in the blog, but I guess it essentially boils down to what is technically possible versus what the players would like to have ("the widest range of possibilities").
My feelings on the matter of spec/class are that I always prefer to play what I like the most, and need not worry about what is "best" or "optimal". I like being optimal, sure, but that comes after I find something to have fun with.
I'm a PvE-oriented player, and I play a tank-specced DK and a heal-specced Shaman in raids, despite reading and hearing numerous reports on how much better any of the other classes fare better. Heck, the hunter alt I leveled to give a hand in raids was BM, which was sub-optimal compared to MM. But I play those classes and spec because these are the ones I have the most fun with, and that's what matters the most.
The trouble is that being optimal matters, albeit differently depending on what you're currently trying to achieve. I'm playing in a somewhat casual guild, where we do not care about "rankings" or optimality - we just have our fun clearing raids in normal modes and taking a shot at a challenge or two in HM or with an achievement. It'd be better if X was a good healer instead of an average one, but most probably, they're doing their best and can't throw in anything better - or they're playing an average spec rather than a good one, but they have fun with it and wouldn't have any with the other. From our point of view, fun is what matters, so we will still welcome that player in our raid like any other player from our guild, from good ones to even bad ones.
But what would happen if, instead, we focused on being the best among our server ? Or if we simply focused on clearing HM rather than normal raids ? Then we would worry about being as optimal as possible, and where 10%, 5%, even 1% did not matter yesterday, it would matter today. But how, on a design point of view, can you make it so two separate specs, while still feeling different, fare equally as good on a given encounter (or worse, on any encounter) ? This is very, very hard to make, and in the facts, we're not talking about just two specs - but about 10 classes with each their potentially multiple specs - so it's not something I see as being possible to pull out.
I've played a DK since the first day of WotLK, and I loved the initial design direction : three specs, three different feelings, three possible ways to play as a DPS, three possible ways to play as a tank. I had fun playing with talent calculators, creating my own specs, testing them out on the field, switching them when I felt a particular encounter could use a certain spec more than another. Of course, those were impossible to balance, and that's probably why that design direction was scrapped.
But, as a matter of fact, I don't really have any issue with performing somewhat less good than another class or another spec, as long as I stay viable (i.e. no "sorry but you just can't tank that encounter with that spec (or even worse, that class)"). I do believe having fun and being optimal, while often pursued together, are two separate goals. One most probably matters more to someone than the other (or you could say that for some, having fun IS being optimal, while that may not necessarily hold true for others), even if if you're pursuing fun, being good makes you feel better (but you do not really care about being "the best"). Thus, given the choice, I'd much rather have the possibility to choose between three different viable specs with one standing slightly above the others, rather than only having the choice between two equally viable specs, or worse, only having one possibility.
In other words, I'd much rather have the possibility to play as a blood-, frost- or unholy-specced tank, with blood standing slightly above as the "optimal" spec, rather than just being given the blood spec as my only option. Sure, from an "optimal" point of view, there would be no difference at all - I'd be playing blood anyway, so why bother considering other sub-optimal options ? - but from a "fun", "variety" point of view, the difference is big.
(part II of my post follows)