First of all, unless you work in a tire shop, you're not doing the "stretching" - the technician is. Most shops won't install a too-narrow tire on a wheel due to liability concerns. So, unless you can mount a tire on your own, whether it "looks easy" or not is irrelevant.
Second, what do you consider "stretched"? We'd need to know what size wheel, what size tire, and what type vehicle.
Third, you have to consider load rating. A smaller tire will have a lower load rating (a measure of the internal construction of the tire) than a larger one. This impacts a tire's ability to not only support the weight of the vehicle in a static state, but also under extreme loads (cornering, braking, potholes, etc) - as well as its ability to dissipate heat.
Fourth, you don't just slap a too-small tire on a wheel and call it good. Handling will suffer. Braking will suffer. Tire longevity will suffer. Your speedometer (depending on the OD of the tire) will be out of whack. Your ABS will be affected (since your wheel sensors feed signal to the ABS computer) and your cruise control may not function (since the wheel sensors feed signal to the ECU).
Fifth, if you get in an accident, and the other driver (or your insurance company) has an attorney that's worth a crap, it wouldn't be hard to deny coverage (or sue your butt off) for operating a car that is compromised by a dangerous tire fitment. Rare? Sure. But who wants to be the guy losing his car because he owes a $20K settlement?
Whether it "looks cool" or not is a matter of opinion (personally, I think it's retarded), but when safety and functionality is affected, a bad idea just gets worse.