Do clients really ask for Bootstrap specifically?
This makes more sense in a context where they are dealing with a lot of freelancers and don't want to deal with 25 idiosyncratic ways to implement a dropdown menu.
http://simpleqode.com/preview/paperclip/1.3.2/ui-elements.ht...
For a programmer I have a pretty good graphic design sense, so I can limp along and build my own components while keeping an overall consistent style. A lot of other programmers are awful at this and I've seen some hideous stuff. Rando freelances are the extreme case, but even if you're stuck having a programmer without UI horse sense do UI work, having a framework based UI toolkit is a huge help.