I think MM said it just right. I will add - if you want good low end response with good volume, combos are not going to cut it. Bam-Bam on drums? No freaking way.
Playing with musicians who have restraint, good dynamics, and a very low stage volume, yeah, you can gig with a combo amp.
The GK MB series of combos are fantastic and can truly keep up with many different kinds of bands.
It really comes down to the tone you want. If you want to hear yourself, you can easily kill the low end through the combo and just focus on mids. It sounds terrible, but now you have a decent monitor and can get through the gig ok.....
Of course combos are gigable, this isn't the 1970's anymore—technology and design has made massive progress and improvements in amplification. I use a Mesa Boogie Walkabout Scout 12" combo. No problems at all with Bam Bam on drums. The sound/tone from this little combo is HUGE!