WTF: Guitar players still seem to love them but I find it hard to see why.
Guitar players love them because of the warmth or distortion they produce when they're run at their limits. They change the waves in a very different way, initially to give a really warm resonant tone, or eventually a really dirty distorted tone. You would not usually use a valve amp to run the PA. The first (well you might have a pre-amp) amplifier in the line is a low-powered valve amp, which you turn right up to the point that you get the level of distortion you're looking for. You then put a mic in front of this amp, and then amplify that distorted sound without using valves. You can get pedals and digital effects that seek to mimic this, but that's classically the way you would want to go.
http://en.wikipedia.org/wiki/Distortion_(music)#Valve_overdrive
Oh, and of course, before people started worrying about their hearing, then rather than using a lower powered valve amp, you'd just run the power amp ragged, ala Angus Young, with a wall of Marshalls turned all the way up.