I always carry a new tube on my commuter. It's faster to just change the tube than find the puncture and patch it. I fix the punctured tubes when I get home and use them on my shopper. Since I don't take it as far from home, if the patch fails, I don't have to walk very far.

Also, every patch I've ever applied, while on the road has come loose.
Just like some have mentioned, the glue tends to hold better if it has a chance to "cure" overnight.