For me, the default answer is "yes" for many reasons:
* I'm not smart enough to handle most browsers well in a short amount of time
* I'm not primarily a designer and these frameworks are often gorgeous by default
* I don't want clients to pay for me to reinvent the wheel, and I don't want them left with my personal framework when there are many, better documented ones available
* These designers thought more than I did about accessbility, and I place a high priority on it
* These frameworks can sometimes deliver a lot of bang per byte, and I'd rather be writing stuff only I can do than wasting time trying to get mine as small and efficient as theirs
