Just to play devil's advocate here: I have played games where customization was very wide and it created more confusion and general angst than anything else. Yes, in that game, the "Paladin" type job could spec up for DPS, but whenever there was a party/PUG situation there was nothing but confusion as to why the "Tank" thought they could play DPS when their job role was clearly tank.

It also created players who repeatedly would say "Well this is how I want to play the game" or "This is how my character is" (role play). It got really stupid really fast and, in those cases..even with the ability to go nuts we still ended up with the community expecting exact abilities or specs in line with the trinity system.