I specify bootstrap or whatever as a starting point because it's important to me to have a "toolbox" of stuff to pull from. Even after the themer's work is done, I need a set of UI elements to design with as my project's design evolves. A really good example of HTML/CSS toolboxes are a bunch of the themes at wrapbootstrap.com. Here's an example one I like:
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.