How common is it for graduate schools to allow admission to be deferred for a year
I think the general trend for deferrals is to make them available, but not automatic—usually, you have to explain exactly why you want the deferral, and what you plan to do in the intervening time. For example, doing a volunteer year (for example, for a Peace Corps-like organization) or accepting a prestigious international fellowship are usually acceptable reasons for a deferral. On the other hand, accepting a job offer for a year would normally not be considered grounds for a deferral.  However, I think this is a policy that differs from department to department and institution to institution, so definitely check with them first!

The reason for this is that deferrals impact two admission classes: the current one, as well as the following one. You can't admit extra students, because you have the deferring students from the present year, plus you're now "short" a student for the new incoming class.