I play in a semi-pro outfit (gigging originals about once or twice a week), and I absolutly play every gig with a miced 40W combo (Fender CVR).
We may not be 'hard rock' but we do use some quite heavy guitar sounds at times.
The fact I have a DeuceTone RAT and a TS-9 in my pedal board should tell you something
Anyway, why people think they need huge stacks is beyond me. The more quiet the sound on stage, the better your monitor mix will be and the better the sound will be out front.
Most engineers I know just give up when they see some teenagers arrive with a huge stack thinking "louder is better". They figure since they won't be able to control the sound coming from that rig anyway, why bother at all?!
Those things might be useful in larger venues, when you don't have a PA, or a decent sound engineer; but I don't know why they are so popular among smaller bands (i.e. - not playing stadiums).
The main reason for not going with a combo is to avoid all the extranious noise that combos make. Because everything is in the same box a valve combo will often rattle and shake and hiss and hum like crazy.
So yes - a head and box is a better design - but why do people feel they need over 50W and a giant 4x12"?
My combo sounds just amazing, is highly controlable on stage and I never have problems hearing myself - even though the volume on it rarely creeps up above 3 (the sweet spot for it breaking up when I play hard). That's what those monitors down the front of the stage are for
We're often complemented on our "hi-fi" sound.
I put this down to our taking the live sound as seriously as we do the recorded sound - balancing everything in rehearsals and noting down settings. Adjusting when we get to the venue for the type of stage (less bass for a hollow rostum etc), working with the engineer and generally not just cranking everything to 11 and hoping for the best as that is "rock n' roll".