There is no reason to have one on a FO or Fowlr tank. Also just because your using RO/DI water that is not going to keep algae from growing in your tank or having bad water quailty.
In a fish only tank where you have a large load of fish and feed a lot, your going to have algae growth in the tank as your nitrates are going to be high. So reguardless if you use RO/DI water or tap your going to have a chance for algae growth in your tank. You also see plenty of reef tank that use RO/DI that have algae growth all the time and have to use hermits or snails to keep it under control, so you see it's more than just the water. In a FO/Fowlr tank, with the load and feeding, you can expect some algae growth. The amount of stuff that RO/DI will take out of the water will not have that much of an impact over tap, specially if the tap is good like NYC water.
If you do water changes and keep your tank clean, not to mention having fish that may eat the algae, it should never be a problem. In my tanks I've been using tap water from years and years and I don't have any problems with algae except maybe what grows on the front on the glass. And a lot of other people that just use tap water don't either. But some people do and that's because of the load and feeding and that can be excepted and using RO/DI wate isn't going to change that.
Reguardless of the cost, high or not, it's not some thing that is need for a FO/Fowlr tank. If the local water is good enough you should have no problems using it in the tank. I think people think RO/DI water is magic and it's going to cure all of your problems but that it not the case.
IS RO/DI water better than tap? Yes it is. Is it needed in a FO or Fowlr tank? No it's not.