It was Wrath that started this bring the player not the class thing, precisely because people felt that in certain encounters they needed to stack certian classes. What they should have done was eliminate the advantage of stacking, but... no.
And I understand this to a degree. Say you have Pete who's a friend, who's been in raid for a long time. All of a sudden, you need more of one class and Pete has to sit because he plays a class that doesn't bring an advantage (Remember, raids were fixed sizes... if you raided Kara, you had 10 people. T5 and 6? 25. Not 27.). That sucks. It's bad for Pete who has to sit and it's bad for Pete's friends who are confronted with including their buddy vs progression. Hell, I had an alt that was in that =situation in BC - my spriest. If we needed it as a mana battery, yay. But if not, her DPS was 30% below other casters and the mana battery role was filled so...
20/20 hindsight says that they should have moved to flexible raid sizes much earlier if they could have technically since then you bring old Pete and so what if he's the 11th person or the 16th... you can still bring him while bringing all of the locks, shammies, whatever.
The other thing we lost with Wrath was the non-DPS utility. As a rogue in TBC, I brought a whole toolkit that wasn't just DPS but was in-combat control (from Blind and Kidney Shot to Evasion, etc). I could use different poisons to do different things. Now? All people care about is where you are on the DPS charts.