I tend to prefer the head/cab combination to combos myself these days. There are several reasons.
1. Moving things around can be more convenient. More pieces, but generally lighter than a combo.
2. Options. You can get a 1 x 12, a 2 x 12, a 4 x 12 or any of many other options when money permits. You can have things with different speakers etc. As money permits, you can get all sorts of cabs and mix and match things. And if you get some other kind of head, you already have come cabbage laying around that you can use.
3. There's no rule that says the head has to sit on top of a cab. At home, I often have a head sitting on the desk in front of me. Makes using the controls very convenient.
4. If you have multiple cabs, you may have one for home, one for your practice facility, one for gigs. Means you're not always having to move everything.
5. Though more expensive, a head + cab(of similar configuration) can sound very similar yet offer the conveniences listed above.
It's true that a 2x12 cab is likely to offer somewhat less volume than a 4 x 12 cab. Speakers make sound by moving air and 2 speakers just can't phyiscally move as much. However, this is only a factor if you're using your amp totally dimed most of the time. I seriously doubt that's true, but what do I know.
Also, don't discount the usefulness of different sized speakers I really like 10's myself. Not as boomy as 12s. Tighter bottom end with less flabbiness. 12s are certainly appropriate for some things, but other sized speakers can sound great. I've always wanted to build something like a cab with 1 x 8" and a 1 x 15" Could sound really interesting.
There are lots of excellent cabs out there. I have many. I often buy cheaper used cabs and upgrade the speakers. Can be a cost effective way to get really nice cabs.
Again, to me the bottom line of this answer is flexibility and options. With a combo, you have it as is. You can add extension cabs, but the core of the sound will be the same. I, personally, really like the flexibility of a head + cab combo.