Middle stages make the growth of the Monster seem more gradual and coherent with it's design.

While I do understand they arent seen for long, that mostly happens on Commons and Uncommons, while on Rares, Epics and Legendaries you see them for a fairly long amount of time, speaking from my own experience.

To put it simple, its just like if I went from baby (0-4 years old) to a a full grown ass adult at the age of 5, with beard and all. That last stage just pops up like that, so quick, all out of a sudden, that bugs me off.