There are many reasons to use Twitter Bootstrap and the main one is an ability to create informative visually appealing website quickly and without a designer. You can save money on the design services but is it worth using Twitter Bootstrap if you already have a designer?
The answer: YES. Actually, this tool will come in very handy for the designer. The thing is that the designing process is unpredictable: today a button may look one way but tomorrow it may turn into something totally different. So, let's put aside a long discussion on what is Bootstrap used for by developers and focus on what it can offer to designers.
The key benefits of Bootstrap for the designer:
1. Responsive web-design. Does your designer draw only static pages with fixed width? In most cases, Twitter Bootstrap allows using such design as a basis for extensible site. To implement a responsive design, it will be necessary to think over scalability of elements, write up styles for various screen resolutions, but you already have the base that greatly simplifies the interface development.
2. Predefined elements. Availability of such elements facilitates work for both you and the designer. It is helpful not only at the initial stage of web development but also later, when the project scales up and there are lots of forms, buttons, links, etc. Unless your designer bothers to depict forms with errors in fields, Twitter Bootstrap provides quite nice set of all elements required to create web applications.
3. Consistency. Often it is one of the biggest designer's headaches. This is especially noticeable in large projects, where one person starts creating the design, the second one continues and the third designer works on it from time to time. Each of them has their own vision, makes their own stylization and fonts. Due to the predefined elements you can get consistency in the entire project. Everything will look equally beautiful.
4. Reduction of the development time. This is one of the most essential points because it does not only allow decreasing the time for front-end web development but also saves nerves of developers. And reduces amount of code!
5. Grid and columns. If you are "lucky" to have design without grid but with some conditional columns, you always can embed them in the existing Bootstrap's columns functionality. This structures the content.
6. Common stylesheet. If the designer relies only on Bootstrap for making mock-ups, then there is no need to create separate pages with the list of elements, as you can see what each element looks like on the Twitter Bootstrap website. This also means a predictable behavior of the elements. Therefore, you know what happens when you hover over, focus, click, etc. Typically, designers do not bother to animate elements, hence the site looks "flat" in the worst sense of this word.
Thus, I think that the Bootstrap is an ideal layer that builds mutual understanding between designer and front-end coder. It is not something to be neglected. The benefits provided by this front-end framework make it one of must-have tools for creating websites.
P.S. It should be mentioned that when a significant customization is required, it is necessary to rewrite almost all styles, so that only a handful of unused and rewritable files remain from the Bootstrap. However, it is difficult to overestimate this framework as a solution for a quick start and work of the designer.