I have been using the poultry nipples for over 2 years now. I understand your concern, as I had the same thing at first. A few things I've learned:
* They DO get enough water from the nipples
* They will rush a watering dish/trough because it is simply easier for them to drink from it
* The nipples are a convenience for US, but an effort for the chickens (it's more work for them to get their water)
In a similar manner, chickens with jammed-packed full crops will rush to eat whatever treat you throw on the ground for you.
Now, in the heat of summer (we had weeks of 100-105F this summer), I will also give them a watering fountain near the shady place they like to hang out while free-ranging. Not because I think they need more water, but because I would generally keep their watering nipple bucket in the run. It would get very hot and while they probably didn't need it, I didn't like the idea of them having to run back and forth from the shady place in the direct sun to get their water.
But I have had the chickens cooped up for weeks, with no other water source other than the nipple bucket, without issue. I've noticed that each bird will spend more time during the day drinking water with the nipples...simply because it takes longer for them to satisfy their thirst than it does drinking from a dish or fountain. This doesn't concern me...I know they are not going to let themselves go thirsty! And the hassle it saves me from cleaning out and refilling the water source every day or two is wonderful.
Now if you live someplace where it regularly gets below freezing, you are going to end up with frozen nipples (insert joke here)! Two solutions to this are 1) bring the nipple bucket into a warm place (such as your home) every night, 2) install an electrical de-icer device to keep the water and nipples thawed.
If you decide upon #2, take my advice and spring for the Horizontal Nipples and get a stock tank de-icer. DON'T use a submersible aquarium tank heater, they are NOT designed for outdoor use...the one I had nearly started a fire! The vertical nipples (the ones than install on the underside of a bucket) just freeze WAY too easily. The horizontal nipples are superior in every regard, IMO, especially when it comes to not getting frozen up. There is a good thread about all this here:
https://www.backyardchickens.com/t/...-vs-horizontal-spring-loaded-watering-nipples