Need? No, we don't need one, any more than we needed 5E or 4E or 3E or 2E or 1E. We could still be playing OD&D today.
I do think it would be beneficial to have one eventually, though. I'm envisioning something like the 1E to 2E transition: The core of the system stays the same, and most old edition material can be easily adapted to the new, but a lot of stuff gets cleaned up and polished and streamlined, and the best splatbook material gets pulled into the core. It would provide the designers an opportunity to address all sorts of minor issues. Certain subclasses in the PHB that didn't really work and needed patching via splatbook (e.g., bladelock, beastmaster); the, shall we say, uneven performance of monsters in the Monster Manual; a couple of feats that need reining in; assorted oddities and counterintuitive bits in the rules.
However, the current edition has plenty of life left in it. I wouldn't want to see 6E rolled out until sometime in the 2020s.