My experience with "modular" is that people either end up bringing both 95% of the time or just one 95% of the time, making the extra money sunk into a paper weight (or the extra money sunk into having two cabs) mostly pointless. People often get their minds stuck in a loop on modularity because it appeals to our minds' aesthetics and other goofy inner workings.
Just my two pence; not judging anyone. If it works for you, that's great.
Agreed, at least for my own situation. Not a fearful, but mine is in a 12/6+12sub configuration and I play both 95% of the time. Even times when I end up only using one I still brought both and just unplugged the 112 to tame down output, get a little more travel in the knobs, etc.
Some reasons for going modular....
1. Folks with back problems, other medical issues, injuries, etc. that need lighter carries, even if it means more trips.
2. Folks that really can use the 1 cab often enough to make it worth it, adding the second for the occasional larger gig.
3. Packing a lot of gear in a small vehicle. The smaller boxes are better for tetris, fit more places.
With respect to #3, that's what I do, carrying rig, smallish PA and all the extra stuff that goes with it in a hatchback, though I've figured out a cab design and packing scheme that will allow me to rebuild it as a single box 1212/6.
Yep, the 12" LF's are a good 3-4db lower spl than the 15's. Using 2 of them makes up for it. It may seem like a wash, but getting away from the technical and into the artsy-fartsy subjective here, things happen when you move from a single woofer to multiples. There is a bit of coupling in the lowend from the multiple woofers. There might be a bit of narrowing of vertical dispersion, focusing more sound forward, but I wouldn't think that would have much if any effect considering the fearfuls 750hz-ish crossover point. Ultimate power handling is increased. There may be some psychological effect in knowing you have more speakers behind you, but, whatever the reason, or combination of reasons, running a single woofer rig and a dual woofer rig to the same measured spl, the dual woofer rig will still "feel a little bigger", IME.