What is so bad about a priori?

I’m not a lover of dichotomies and unnecessary philosophical distinctions. But I can’t see the motivation for abandoning the a priori/a posteriori distinction (or something in the vicinity of it).

It seems to me obvious that there is a difference between understanding and mere knowledge. There is difference between me understanding Pythagorean theorem (that is, understanding the relations depicted by it, and why those relations hold), and mere learning it by heart.

Maybe there is something about the terminology and its historical burden that alienates some people from a priori/a posteriori distinction. Would they accept the distinction between understanding and mere knowledge on another hand? If they do, what would they make of that distinction? Is it of qualitative or just a quantitative nature?