That thread actually just consists of nyghirazi and keyboardclass bashing each other (although I feel nyghirazi has more to contribute than keyboardclass)
Anyway, yeah it's a bad habit. A particularly difficult passage made me collapse the last joint on my pinkie, and it ended up making my pinkie sore. Collapsing the joint places incredible stress on it while not collapsing it supports the finger tip from the finger.
The reason you are probably doing this however, is because black keys are smaller than white keys. I noticed when I was playing Chopin's Black key etude that my fingers were pretty much almost flat. How I managed to play all the notes I'll never know. Anyway, black keys require a bit more accuracy with your fingers and flattening them out increases the chance of hitting the right note (although this doesn't work for white keys because there is no empty space between the white keys.
But you will be much better off if you fix this. Take things slowly and make sure your pinkie is working properly.