Pyros can just push heavies away, so they're not terribly good for rushing sentries. Demos are best for that.
The real problem, though, is newbie medics who won't even heal me when I'm playing soldier. They just keep healing the heavies, even though the newbie heavy's not going to turn around even after the pyro behind him sets him on fire.
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Oh, a big machine with flashy lights. A big machine like that has me written all over it. Well, okay, it doesn't. But give me time... and a crayon --11th Doctor