My current project for a client has dozens of pretty basic CRUD screens and a few slightly more complex screens. Its reasonable to look at, and scales to a mobile device, including a side navigation bar that slides in/out on small screens.
I wrote about 10-20 lines of CSS.
Even though it's expressly an "admin" UI for the app (which has no public web UI) they would not accept an app made with no stylesheet and just a page of form elements. It would be practically unusable without any CSS, and the time it would take me to write the CSS for just the navigation bar OR just the pagination system OR just the reasonable form layout on multiple screen sizes, dwarfs the amount of time it takes me to download bootstrap and copy the dist folder.
For a public facing website - I think bootstrap is terrible, and I'd rather see something like Bourbon used, where semantic markup is then styled.
This isn't about that. It's about getting a usable interface that takes almost zero time to implement.