skeejok on rereading your reply I now understand that it basically boils down to "spy (and scout) becomes harder when ppl use higher fov". Well, if you REALLY wanna talk about balance: The game IS linear (I think you mean symmetric), unlike a game like Natural Selection, because both teams have exactly the same options. So any detriment to a class works for both sides. No imbalance there.
Yes I meant it's asymmetrical, not nonlinear, as opposed to something like Quake that isn't class-based. A detriment to both sides is technically equal, but it's still a detriment to a class. For example, you can't say that Scouts should have 300 base health instead of 125 if your argument is "Well, both BLU and RED will get this buff, therefore it's balanced", because it's equal, but not balanced.
Again, I'm not at all arguing that the max FOV should be 90. In my opinion it should be 110. I'm just arguing against an uncapped value.
skeejEdit2: Also, a completely different point: the balance of FOV doesn't discriminate against people with worse hardware, unlike better fps/sound/etc. Anyone can run TF2 with vfov 75-90 (I'd be in favor of upping this restriction) and in any aspect ratio, regardless of hardware.
For the record, FOV directly impacts performance. Higher FOV = lower performance, on a pretty big scale. There's a reason consoles have such a low FOV - it isn't because console players sit on the couch, it's because consoles are weak and so they lower FOV, which they can afford to do because players sit on the couch, in order to gain back some performance.
That being said, FOV doesn't cause much of a performance hit in TF2 for the same reason that graphics configs don't really do all that much in regards to performance, in the majority of cases. TF2 is not GPU-bound, and increasing the FOV is (mostly) only going to hit the GPU. Performance won't change much in TF2 because of this.