It's true that when you're singing unaccompanied you can concentrate more on the lyrics than if you are playing an instrument at the same time. It's also good (as George P says) to be able to do appropriate hand movements, etc., for some songs, which you can't do while playing.

Yes, a whole evening of unaccompanied singing sounds as if it could be boring, but with the right artists I'm sure it can be good. And I've endured a few dire evenings of accompanied singing :-)

On balance a mixture of both (which is what I usually see) is better, IMO.