I don't think it's always harmful if you use it as a device to trigger a verse occasionally. Reading the whole song off the page is more problematic because the focus isn't on communication.
I think Dick is right, it's best not to use it if you actually don't have to.

What I find problematic are song sessions where people bring out songbooks to sing from. This takes away from the listening to one another particularly if people are singing together.

Frank