I like the idea, at least in principle.
I've had to replace belts and tensioners on many higher mileage cars I've owned. At least this way, I'm only replacing the belt, and not a tensioner (usually cost about the same as two standard drive belts) as well.
Although, if it's a one-time use belt (you have to cut it off?), and it's time to work on something else in the belt drive...there goes half the savings.