There are two flavours of two-bolt SPD MTB cleats: The usual SH51 cleats which will release if you rotate your foot (in either direction, but the crank usually gets in the way if you try to do it inwards). Shimano also make SH56 multi-release cleats which will additionally release if you pull firmly upwards. The difference is fairly subtle if you aren't comparing them side by side.
That's a potential advantage if you're new to clipless, and probably handy for mountain biking. Sounds like a bad idea for recumbent riders used to pedalling in circles (I can imagine failing to get your second foot up in time while starting off, trying to complete the first revolution of the pedals one-footed only to have the pedal release and cause you to bash your Achilles on the front wheel, followed by falling over sideways due to lack of momentum), though I've never actually tried them.