There was a huge thread over at AA about this, I'll post if I can find it. Yeah it takes the guy a while to warm up, but when you leave it running your killing tube life and also leaving yourself open to a fire risk (i assume more of an issue with vintage gear, but the same ideas apply in modern gear so maybe not). The last thing you want is for something to go bad (tube, cap, whatever) in the amp and have it run away when you're not around.
From what I've read SS rectification in tube amps is particularly hard on amps at turn on because the plate current is nearly instantly there before the filaments have warmed up. Mine uses a tube rectifier so things are a little easier on the amp, works like a built in standby switch. The amp starts up as >> heaters come on, tubes begin to warm, when the rectifier has warmed up, DC plate current is passed, then bias voltage comes up (delay is varied by the size of the caps in the circuits that need to charge before passing current). I think thats right, but the basic point is that until the tubes are warm, they're not seeing any plate current because its not getting through the rectifier.