In my mind, it makes more sense overall to have two 2x12's instead of a single 4x12.
Firstly, they're easier to carry and transport. Imagining a situation where you have to carry cabinets up a flight of stairs, it would be much easier to take each 2x12 at a time than to carry the single heavy 4x12.
Second, in situations where you don't need the power or sound of a 4x12, you can simply take one 2x12 with you. If you only have a 4x12, you're stuck with that.
Third, you can create cool surround sound/stereo situations by placing the 2x12's in different positions on stage.
Fourth, if you wanted, you could have different speakers in each cabinet, so you can choose which speakers you want for each application. In a 4x12, you always have to use the same ones (unless you swap them out, but that's annoying)
Fifth, but not least, two 2x12's stacked on top of each other look damn cool!
The advantages of a 4x12 are that it takes up less bulk room (it's taller than each individual 2x12, but doesn't take up as much space as two 2x12's next to each other) and they're cheaper to buy...
Anything I've missed or failed to realise? Is my bias too obvious?
Firstly, they're easier to carry and transport. Imagining a situation where you have to carry cabinets up a flight of stairs, it would be much easier to take each 2x12 at a time than to carry the single heavy 4x12.
Second, in situations where you don't need the power or sound of a 4x12, you can simply take one 2x12 with you. If you only have a 4x12, you're stuck with that.
Third, you can create cool surround sound/stereo situations by placing the 2x12's in different positions on stage.
Fourth, if you wanted, you could have different speakers in each cabinet, so you can choose which speakers you want for each application. In a 4x12, you always have to use the same ones (unless you swap them out, but that's annoying)
Fifth, but not least, two 2x12's stacked on top of each other look damn cool!

The advantages of a 4x12 are that it takes up less bulk room (it's taller than each individual 2x12, but doesn't take up as much space as two 2x12's next to each other) and they're cheaper to buy...
Anything I've missed or failed to realise? Is my bias too obvious?
