I personally always missed this feature when doing custom window size management systems. Although it indeed looks ugly when the stretch is too big, it's very useful for when there's those 20-50px missing at each side in a fixed sized layout.
If you stretch something by 5-10% maximum the stretch is practically imperceptible and the result is much better then that small gap that is too small to seem done on purpose, and look like a glitch or a miscalculation instead.
For being really useful it needs to be controllable during runtime though, since applying it at all times just looks horrible.