Since reversing entries are optional, is there any true harm in not doing them?

There's no harm whatsoever, as long as you can still keep track of things properly. Reversing entries are more of a convenience. There are various reasons reversing entries can come in handy, and depending on that reason, it may or may not be important to do it.

The thing about reversing entries is that you need to know why you're doing them to begin with, and therefore whether you want or need to do that particular one. If you've learned how to do entries based upon always reversing things, it might be confusing not to do them. If more than one person is handling the books (say a bookkeeper at the company and the CPA firm), then sometimes it's best to do them consistently. (If you're a bookkeeper and the CPA said to do them, then do them.)

Really, this is situation dependent and person dependent. What I do might not be the same as what someone else does.

In the long run, the most important thing is that every account ends up with the proper balances by the end of the period, regardless of how you got there.

Did you have something specific in mind?