If you disconnect the hubs and leave the front driveline engaged the front axle and shafts will just be spinning with no load, not generally a good thing.

As for using your spare, there's no real issue with the driveline that would cause problems with limited use as is. You'd have greater steering and handling problems than would be caused by a little torque-bind from the slightly different tire diameter.

Some in the past have looked into disconnecting the front driveline in the hopes of increasing fuel economy, so I figured that might have been your motivation.