Amen Neil.
Up until we had temperatures in the teens, I had supplied both nipple and conventional water to chickens. The only reason I started offering the conventional unit was because I had a chukar in the mix and it wasn't caring for the nipple.
My chickens take their water from both sources fairly equally just fine. When they only had a nipple to go to, they were guzzling water from the nipple. I know this because I watch them DAILY (yes daily... lol) from a netcam throughout the day, guzzling away. This has provided a lot of insight to the very same worries some have expressed over nipple systems, especially during the hot summer when hydration is very important. The cameras give me some valuable feedback as to how the chickens respond to various stimuli and their daily routine and cohabitation.
Since my water source for the indoor nipple exists outside with lots of tubing (= instant freeze), I pretty much plan to retire it until next spring and leave the conventional unit for the time being. I'll likely add some kind of compact nipple contraption on the inside as soon as I figure out a stable yet easy to fill/clean design.