I've also never heard this. And IMHO this is very wrong way to think about it. Bootstrap is a comprehensive set of well designed and coordinated UI widgets and paradigms. And it's all customizable, so you don't have to get stuck with that "bootstrap look".

Because it's way faster to test ideas and also look reasonably well. However, over time it will get in the way and you'll be better off extracting the stuff you actually use. At least it was my latest experience with it.