Also, how much of even a sablizer role do the rear delts play in a row or DL? My experience has been that stablizer muscles play a larger part in press excercises (for obvious balance purposes) than in pulling excercises, such as rows and deads.
I feel that doing upright(bend over just by a few degrees rows with a shoulder width grip works the side and rear delts just as well as presses work the front.
Use a shoulder width grip because the very close grip that most people use focuses on traps. And you should bend over a little to hit the rear delts a little more.
Trust me on this. my rear and side delts were really lagging until just a while ago. And during that time I did not do any side or rear delt rasises but once i started doing those again they became allot easer. So i know that this exercise works very well if done properly.
Heavy compound movements are simply more effective (IMO) for adding mass. I don't know about you, but my rear delts contract pretty hard when I'm pulling a row. They might be considered stabilization in a deadlift type movement, but not in a row (correct me if I'm wrong).
Another reason, and this is pure theory on my part, is the fact that you're making weight increases so rapidly. You might go from a 100 lb row to a 200 lb row in the time it takes you to go from 15 to 20 lb dumbells, for example.
Debaser, I suspect the reason for growth stimulation from compound movements is more related to hormone relase due to a much larger mass of muscle fibers being recruited during each set.
... this is not to say that additonal fibers cannot be recruited for further growth in the localized muscle by isolation movements though, its just that you are likely to see faster growth in all the muscles involved in a compound movement than you would by simply doing isolation movements.