This idea is something I've thought about for a while. Mainly because seeedstudio's $10 PCBs are 1.9" square and the arduino shield needs to be 2.1" to span both rows of headers. That extra 0.2" raises the price from $10 to $25. I considered an adapter that I'd only have to make once for unlimited mini-shields but it will be annoying to have the extra layer just to use non-standard shields.
The other thing I've thought of is making an oversized adapter shield that will have spots for 4 mini-shields, replicating all or some of the IO multiple times, so as long as you're careful of which pins you use on each mini-shield, it should make modular design easier. That still misses the whole point of wanting to do quick cheap easy shields though.