Does anyone of Web Developers use WordPress themes to build websites for you clients?

I agree with Abhijeet on a lot of things.
Another thing I will add is that it depends on if your client wants to be able to change the content easily (i.e. image gallery, update copy).
I built a site for a client recently where they wanted a very customized look, a calendar widget, a price quote generator, contact form, and some other things. It probably would have been faster to use a WP theme and find the appropriate plugins for each bit of functionality, but then I may have had to battle with the default styles and javascript to achieve the custom look. So I opted to build it all from scratch using plain old HTML, CSS, JS, and a little bit of PHP for the contact form.
There is nothing wrong with using WP to build things out like that, it can just be a matter of preference.

Thanks for your answer!