I use both because I introduced a chukar some time ago to my girls and it didn't appear to know what to do with a nipple.
I have a nipple and a small waterer in the coop, and a nipple in the run. The chickens use both now, but in the past they had no problem what so ever with my 5 gal nipple set up and drink quite a lot. The amount has decrease with the addition of the waterer but they still use both. I know because I can spy on them remotely.
I'm tempted to go back to nipples only after observing (indirectly -- heard the nipple being used behind me when all chickens are in front) the chukar using the nipple outside. It sure would cut down on my frequent water changes.
My concern with a pressured nipple system would be with freezing... It's probably ok, but I would not want to come home after work to find that it's been leaking something fierce all day. I plan to introduce an internal waterer to replace my current nipple during the winter to reduce freezing the water line.