I've used undersized tubes for easily hundreds if not thousands of miles with little real harm. I don't think the stretch ( and subsequent loss of durability and ability to hold pressure) is a real problem. None of my bikes experience any loss of pressure beyond what is to be expected. I just top them up every week or two, whenever I remember, and they're all fine. The tube isn't a major barrier to puncture anyway, so it shouldn't affect the rate at which you get flats at least.

Neither has my ability to repair a puncture been at all impeded. I have a bike with 23mm tubes inside of 45 mm tires that has been patched 10 times (at last count). If it weren't for the occasional pinch flat, or hole right on the valve stem, I don't know if I'd ever actually buy new tubes.

However, if you really disobey manufacturer recommendations and use a wide tire on a narrow rim with a narrow tube, you do get more flats (ask me how I know). The tube around the stem seems to stretch out and wear thin, and I got a string of holes right around the reinforced grommet. Switching to a tube that was closer to the right size helped out a bunch with that particular problem.