Bootstrap raised the lowest common denominator from a 2 to about 35. You can still put together a horrid UI with bad spacing and mismatched fonts, but you have to go out of your way to get UI quality below 20.
If you have the skills to write 90+ UIs from scratch, you probably get little out of bootstrap, unless you're rapidly prototyping and need a 65% UI in four hours.
It's a great tool that improved both the minimum level of the web and the efficiency with which good UIs can be built.
If you don't need it, that's great, you're a highly skilled specialist. But it is still an extremely valuable tool for large swaths of the web.