Baby greens have compacted flavor that makes them more sweet and tender. They are used in many meals but usually they are used as garnishes. An additional idea is that compacted flavor = compacted nutrients. Thus, if you eat one micro mustard green, you get the same amount of nutrients as you would get eating a whole mustard green.
Only one study that I know of has been done on that however. You can find out more about microgreens here: http://www.freshorigins.com/microgreens.html