I think it depends on the water quality. Good quality tap water (considered healthy for humans) has some naturally occurring minerals that ARE good for people and some plants. Some species of plants and animals (orchids and fish, for example), are really sensitive to minerals, and so tap water, especially unbalanced tap water, could cause issues. I have a good friend who is an orchid connoisseur, and he intentionally adds tap water to his RO water in order to add minerals back into the water, but just enough that it doesn't burn his sensitive palnts.
I agree RO systems are pretty wasteful.
@jamest0o0 has a way more sophisticated RO set up than I did. I'd just run my attached to my garden hose, and I'd use the extra run off water to water in a separate bucket that I'd use to water my veggie garden. It's a slow process to fill up a gallon of RO water... like 20-30 mins per gallon. That's a LOT of wasted water if you aren't recycling it somehow.