The sad thing is that there is a lot of research out there showing how shy kids really get the short end of the stick, at least in the West.
I think very few behaviors have adaptive value. And those that do are purely by accident. The funny thing is that you can put an evo. psych spin on almost any behavior, however it gets kind of funny when you do it with humans and not animals (well non-human animals).
For example, if a little girl kicks a little boy in the sack, and these two children were wearing, figuratively speaking, monkey suits, an evo. psych might see this behavior as a social gesture in which the female disapproves of the males traits and wishes to decrease his chances at mating by lowering his sperm count. Maybe that actually happens, but she only did it because she saw it in an Adam Sandler movie.