I do that too. The last time I got around to patching them I had something like 25 of them in various sizes. I had them hanging all over the garage the weekend that I patched them. Now I've got a really big bag of spare tubes, which is nice. Also, I have a lot more confidence with a tube that I saw hanging fully inflated in the garage for several days than I would with one that I patched by the side of the road. Not all of my patched tubes held air for multiple days.

I still carry a patch kit in case the spare tube goes down, but I almost never use it.