I'm okay with the crits, but also against them, but that's only because I love it when the crits are coming from me and not if from someone else. That's always part of the game.
As for being opposed from crits, it's only because I don't necessarily find luck to be a very essential part of Arena. Every other mode seems reasonable, but not necessarily from arena, only because you get one life and that's it. A soldier can show up with a crit rocket in from of a team in a corner, and take out nearly all of them with one shot. I would find it fair however, if these crits were coming from a kritzkrieg. I say this because I notice some teams actually play without the use of a medic. Some only have the ubergun, and some have the kritz. With crits removed, it would make the Medic class, as well as the kritzkrieg a bit more essential to victory. Crits from the backburner, knife, headshots, or the KGB are reasonable, as that's what the weapon was made for. As for other weapons, it''s a luxury to crit.
On the side of keeping them, it gives the game a bit more awareness, especially of your medics and dispensers (which I'm glad stay up after death). Everything else that makes crits good has already been said, so I'll leave it there.
Personally, I think the biggest problem with our arena server is actually the amount of slots we have, but in my opinion, I think we should give no crits a try and see how it works. There's no more Custom tab, so why not?