I used to work for a small personal training studio, which offered only personal training. But for those clients who were still using personal training sessions, the studio was available to them so they could use the bikes, treadmills and elliptical machines even when it wasn’t their day to train. I think it was a great way to keep them active and keeping them from joining another gym just so they could use those machines.
I agree with Michael that the spin bikes should only be used under supervision, for both safety and liability reasons. But to allow access to other areas in your gym, as Harris suggests encourages activity and keeps the clients a part of your facility. It goes without saying that you need to make sure that your liability insurance adequately covers you when clients (and staff) use equipment unsupervised.