Think about sites like Hacker News or Craigslist. They are arguably not very "tasty" designs, but they are simple and they work.

That's an important point to digest. Just because something looks nice doesn't mean it's effective. It may be the opposite.

Sure, the website looks "nice", I guess, but is that the goal? Or is the goal of the site to get sign ups, generate referrals, make money, etc?

When you're done plugging in Bootstrap, does the design serve your goals? Or does it just look nice? Have you saved time or merely punted the design farther down the road?

Again, I'm playing Devil's advocate because I do see the value in Bootstrap. I just rarely hear any contrary opinions on Bootstrap and it's important to understand (or at least discuss) the trade offs in using a canned design.