I don't think you can possible "stretch" a chain so much that it breaks.

I've had master links come off if I reused them too much. The only time I've had a chain break is when I tried to reuse a chain that had actually been twisted into a pretzel when the derailleur came off the mount, I straightened the chain, about 400 miles later it actually broke.

I don't even bother carrying tools anymore. I've been riding mostly daily for 12 years and about 35000 miles and I have yet to need a tool apart from a tire lever and a pump (which I still carry) - I don't even need the tire lever on the road bike, the tires just come off easily by hand.