There is something I keep pointing out when inevitably the conversation turns this way: Bootstrap (and Foundation) are
UI widget libraries
, not just prototyping helpers. They are the missing rich widget set that browsers should provide by default if they were proper application runtimes and not the patched together mess they have grown into. A good comparison would be Sencha ExtJS, but for markup instead of full JavaScript: you have layout elements (the grid), a set of reusable controls, and some helpers (typography, etc).
A lot of web applications -- line-of-business but also consumer-oriented -- should NOT be visibly unique, and styling your own buttons not only is a big waste of time, but also reduces usability. Look at the WinForms/Cocoa/GTK environments to see the value of a single visual language in action (including their respective UI guides that help with a consistent experience). The popularity of Bootstrap tells you the need is still there -- for both the millions of developers who need to get the job done without reinventing too many wheels on top of broken hypertext, and for the users who appreciate not having to figure out a UI for the millionth time (just ask your boss/customer what they think of your boring Bootstrap app, they probably love it).