I honestly can't figure out why anyone would need a 100-watt amp at all in this day and age. If you're playing a gig large enough to need that kind of stage volume, your amp will be miked anyway, not to mention that 4x12s create all kinds of problems for the sound guy.
I use a Marshall rig for backline - a stereo power amp and a 4x12 each side...
sounds great...
couple of things that occur to me - and this is just my opinion right...
- 4x12's do not give sound engineers problems. they've been the bread and butter of rock guitarists for around half a century..
- when you play on a big stage you really do want to move a little more air than you would from a 1x12.. yes you have monitors etc.. but it's not the same..
- don't forget that part of going to 'see' a band is actually seeing them... if you play rock / metal, and your backline looks like Manhattan, the band looks right on stage..
seems like an odd thing to say but it's true... lol..
if I ever recorded mic'd [which I don't btw] I'd have no probs with a 1x12 if I thought it produced the right sound.. but I could never walk on stage with one..
haven't done that for more than 20 something years.. even if they sound sweet they just don't move enough air and will thin out in a big place..
yes it'll get mic'd and come out of your monitors but it'll never feel the same in terms of air pressure and sheer grunt..
walk onto a stage in a venue that holds a couple of thousand with a 1x12 combo, be stood 20' or more from it and you'll see what I mean..
In a bar or small club that holds just a few hundred and they'll be fine - I know this cos I've done it..
but in a big open air place you'll need more.. <-- edit: let's say I prefer more