I'd want them to add it (or someone make a script for it) but I do realize that it may be a fringe scenario. Personally, I'd prefer doing anything in a single app that can be done in the single app. I don't like doing much in photosphop/photopaint.
The real benefit is to make it easier to package textures from multiple models into the same texture. Yeah there are pros/cons...
The scenario I'd like it the most is like this:
I've made WWMT so that it can export multiple models at once and using the disp tool you can export them into source al at once. If I have ten models that make up the wall/junk/props of a little set and they all "feel" like a single model (because the textures really go from one to the next) then why not just unwrap them into the same UVW and export one texture?
One obvious convenience of this is that if you do decide to edit the colors later in photoshop, then the levels, hues, etc propagate to all your models all at once instead of having to go and replicate the changes in all the different textures. You can still do this with the solutions mentioned above (compositing textures) but really that's the whole thing... the compositing phase of this scenario is annoying drudgery--the kind of stuff I prefer to avoid
Anyway, it's personally something I'd like but as now been pointed out enough, it's not a big deal to do it... it's just that it's inconvenient (for some of us). But it's not inconvenient enough for me (personally) to tackle yet... despite the fact that I've wanted the feature for a year or two. I looked for a ready-made solution some time ago and found none (well there were a couple commercial solutions--but I didn't feel it's worth paying for).
For example, check out Flatiron