Bootstrap provides such nice styling "out of the box" that often just including* the Bootstrap assets will make an otherwise plain, but semantically correct, HTML document much more attractive.
There is a school of thought that too much polish in a mockup will bog-down design sessions because the customer will be too concerned about the perfect shade of blue. For this reason, many tools produce mock-ups that look like cartoons and white-board drawings. So the pro's and con's of applying Bootstrap-level polish are worth considering.
*--one of the more simple ways to "include" these assets in your mockup is via a CDN. See http://netdna.bootstrapcdn.com/ for an example.