As I was thinking about this, a never ending string of stacking uShields could get a little tricky amp wise. Please correct me if Im wrong, but the blink-m uses two of the arduino (analog) pins as source for 5v and ground. This will only be good for 40mA correct? Stacking them is an open invite for some dead i/o pins if you ask me. Much beter to have the convenience of sticking on a little piece of kit that gives some functionality (such as a bad ass blinky light OR 3axis sensor, etc) and when you need the kitchen sink you can whip up the breadboard with the appropriate wiring. The trade off of convenience versus full functionality rather than trying to be the be all end all.
This is where the ushield shield comes into play. It provides the power from the comparatively beefy 5V line to a number of little 4 pin sockets.