What is the purpose of developing the raid if you want us to just join for the last boss?
They don't want us to join us for the last boss, but they do want us to reach the last boss. People leave a LFR group for whatever reason and they have to be replaced for the group to progress. And again, since 5.1 people seem to get into fewer x/3 groups, so that would be counter productive if Blizzard just wanted us to kill the last boss.
When they wanted to encourage more tanks into LFG the enticed them with extra loot sacks, for us they poke out our eyes and force us to do what they want.
The issue there is incentive. I'm doing LFD for the Valor, I'm doing LFR for the loot. So I stick to the end of the LFD. Sure the bag is nice, but the flasks are so easy to get that it's not really an issue if there aren't any flask reward.
EDIT: The instant queue for tanks is the main reason why I join LFD as tanks, the value of that outweighs any gift bag by far.
But since I'm there for the loot in LFR, why stick around after I have killed what I need to? Especially if I have more coins, or another spec that needs gear? I could be mean and just respec, meaning I would stay in group and under perform since I'm not doing the role I'm supposed to, but still help out.
As for coins, sure you could change it so I could spend all my coins on a single LFR boss without killing him multiple times. But after I have done that, I still have no incentive to stay.
So I really don't see how a gift bag helping out on this issue, because those who wanted a quick reward after a LFR had a good opportunity in last patch to join a 2/3 do a quick kill and get some valor. And still Blizzard deemed that there were too many unfinished LFR.