Never put all your eggs in one basket, but I do concentrate most on one site. Trying to make each of them exceptional is spreading yourself too thin. Make sure all are indexed and growing a little all the time, and concentrate on one to make perfect.
The search engines like to see sites grow “naturally”. A little new content here and there, a new link here and there, google et al like it. So that’s what to do with your “back up ” pages while you concentrate on making $ with your favorite.
That’s just my take and what has worked for me.