the question is, does it ever cross the line between impractical and destroying certain functions of the animal?
I know what your getting at, and I'm trying to think of an example...
But animal "function" is a bit vague, too. It seems to me that there are many species with female mate choice where the males have given up the "function" of camoflaging pigmentation patterns in favor of insanely bright colors. (The females by comparison are drab, retaining the camoflage "function" that the males have lost).