Back in the great days of Team Fortress Classic, when superb maps shared names with a type of Durex condom (Avanti), the Heavy Weapons Guy was oft-criticised for being the n00b class of choice because it seemingly required absolutely no skill at all.
Every class in Team Fortress requires skill, thought and tactic. Medics must learn to follow teammates while remaining undercover; Heavies must use time-management to allow themselves time to retreat and manoeuvre; Spies must devise ways to pick off enemies without being detected; and Demomen must intelligently monitor remote detonation systems and destroy buildings from afar.
The Pyro just runs around aimlessly blowing fire at people. One 'skill' of the Pyro is the 'hide around corners' technique, as the flamethrower does more damage at point-blank range. But I'm sorry: since when was waiting for an enemy to get close to you a skill? Oh, that's right -- it's not.
I'm not saying Pyros are completely useless, but they're in the same sort of league in gaming as Paris Hilton is in music: no skill is required if someone else is doing all the hard work. Paris is the pretty face 'singing' songs other people wrote and produced and the Pyro mindlessly kills people while the rest of the team works together to coordinate attacks and powerful defence tactics.
The only truly great thing about having Pyros on an opposing team is that they're twice as much fun to kill. In fact, I'm not sure who I'd enjoy seeing blown up with explosives more: a Pyro or Paris Hilton.