Encounters need to be designed to let people play the class they want to.

[Jeffdogg]

http://www.youtube.com/watch?v=sVlLSzCff_o

Nope you can use whatever class you desire to ,
above Video of "Doctor Mog" prove that it's possible to do all class run.
As i always say that "other people just prefer to use some certain classes because it's easier"
Yes , you can use DRG in AV , WAR on Ifrit , MNK on Moogle and whatever class on Chimera
but party leader and other people in the party may not welcome you if you're not in LS party
(pickup party sure want to use the best strategy & party for the success run)
I don't think this is a game nor content fault

I agree, its like on FFXI when it came to CoP missions. People get it stuck in their head that only certain jobs can complete certain missions when that wasn't true at all. All classes can do all fights and missions, the problem is people listen to others and read walkthrus that make people think it can only be done with certain classes which is complete rubbish..