Because everybody running around with spiked models so they can see players through walls is fun?
Honestly yes. More fun than DP is right now anyways.
TF2 has server-side setting to disable custom skins.
sv_pure 0 (all custom skins allowed)
sv_pure 1 (player skins allowed, weapons not allowed)
sv_pure 2 (no custom skins allowed)
I've read that there are also restrictions on the size of the models. This is to prevent spiked models.
I think you would be surprised. Tons of people use custom skins/models in TF2, and they don't do it for an advantage.