People have set up eef systems using tap water, and in a few cases they really don't need an RO/DI unit. That being said, I would consider a good RO/DI unit so critical to maintaining top quality water that I'd rate it as extremely desirable. They are not difficult to set up or use, and you will know the quality of the water you are using. All to often water obtained from your LFS is not that good.
Yes, RO/DI does produce waste water in the process. This doesn't mean you need to throw it out. You can use it for a lot of other things, such as watering plants, wasing and so on.
Yes, you will pay for all the water consumed, but that is still far less that what the LFS will charge you for water, even if you factor in the cost of the RO/DI unit itself.