The closer the curve of a fretboard is to the curve of your hand, the easier playing will be, but a flatter fretboard should allow a player to play faster and perform better bends.
To answer your question, most people benefit from the ease of playability from a radius, even if only a slight radius, so most companies build rounded fretboards, but from what I've seen flat, or flat-ish, fretboards aren't really THAT uncommon.