I suspect that one way Adobe decides new features is to watch and see what the third party folks are doing, but when they add the function, it seems like, even after a few releases, it still lags behind the thrid party apps. Noise, sharpening and HDR in PS are still not on a par with the third party apps.
I still don't get why they even try to compete with the small guys in various niches like stitching, noise reduction, now DoF stacking. For me it looks like a wasteful use of resources at Adobe.
I can very well imagine how it happens. Some bright engineer has an idea "hey, we could do DoF stacking using the align layers capability". Then John, or whoever has a go on define, ends up saying "yes, do it". They guy starts by doing it as a side job overtime, but then they find some issues and have to allocate real resources to finish the job. Since they are under-staffed, as all software developpers are, they end up giving up half way through an release something that works only so so...
I don't mean to be harsh, but personnally, I'd rather have them release quicker the 64 bits support on OSX rather than all these half cooked functions that no serious users will use anyway.
Photoshop should be seen as an image manipulation platform, like an OS. It should focus on stability, speed, capacity and core image manipulations.
Cheers,
Bernard