Hate 'em. Although I LOVE killing a grate-camping demo.
Grate-camping demos contribute to the most fundamental problem with 2Fort: turtling. If we really need defense, then I don't mind a demo stickying the grate. But otherwise, he needs to help push out and get us back on offense. A demo that does nothing but camp the grate, to me, ranks up there with medics that spend most of their time healing snipers and heavies that stand around eating sandviches.
Nothing is more frustrating than being on a team that has 5 engies, 4 snipers, and a couple of demos doing nothing but camping the grate or the courtyard. That happens more often than it should, and it leaves only 5 people on offense. And really, you need a couple of medics to have a successful offense. Now you only have 3 people playing purely offensive classes. That's without the team having any spies.
So demos, come off the grate once in a while and help get the offense going!