Inner tubes are used as patches extensively around the world, but there are issues.

Pete mentioned getting best results using a tube that doesn't have to stretch too much to fill the tire, and I believe this is doubly important when an elastic patch material is used.

As I explained a while back, if the patch stretches, and the bond isn't fierce, the tube itself starts to pull away from the patch as a flat, planar surface, starting at the original puncture hole. This was the failing of the original "Speed Patch" product, which was made from what apparently was a very thin microcell (closed-cell foam) rubber sheet. These patches failed reliably within a day, always starting at the hole. By the time the post-mortem occurred, you could see the circle where the talc had contaminated the adhesive until the circle reached the edge of the patch (at which point the air speedily escaped).

I really test the limits sometimes with my quartered patches, which have only one of three edges "feathered". These are less tolerant of every patching variable it seems, yet can be lifetime-reliable if the tube-sizing and bonding issues are considered. We have goat-head thorn infestation here, so the tiny holes are prime candidates for micro-repair and you can get about 25 repairs out of a single Rema-type patch kit.