Combos are fine for gigging, unless:
a) the band is way too loud on stage, which is unfortunately often the case, or
b) the PA and the rest of the band's gear is not up to the needs of the gig, which will often include sending the bass FOH - but not every time, depending on volume/style of gig/room et cetera.
If the band is stupidly loud on stage, and/or you're trying to fill a huge room with bass and no PA support, then combos are not fine for gigging. In fact, good luck sounding even halfway decent both on stage and out front in those circumstances with ANY rig - that's a total crap shoot.
As for why one would choose a combo over separates, given the range of stuff on the market - well, I for one actually prefer the combo format in some ways. For years now, I've used a single 210 for quite a few of my gigs, just for on stage sound with the bass going through the PA. I like to have the 210 tilted back in those circumstances. Doing that enables me to hear my bass loud and very clear in the on-stage mix without overpowering the rest of the band. When I was using a cab + head, I had to carry both, plus a stand for tilting the cab, and then think about where to put the head on stage, as the top of the cab was angled back and the head couldn't be placed on it. Think about it - with a 210 cab and a head, where do you put the latter in an accessible place on stage if you can't put it on top of your cab?
With my Markbass 210 combo, the design enables me to tilt back with no stand and no issues about where to put the amp. Easiest stage setup I've ever had, in one box that can be carried with one hand. Of course, the fact that the sound that comes out of it to my ears is exactly what I want is all part of the satisfaction equation, too. If it wasn't, I wouldn't use it.
I know some people prefer being able to mix and match heads and cabs - fine, I can see huge advantages in that. But I'm just posting in response to those who wrongly say stuff like "There's no real reason to ever prefer a combo over a comparable separates rig". We all have our own reason for choosing what we use - lots of them - and my own reason for preferring a combo, as described here, is unambiguous and totally valid. I'm sure that goes for others, too.