Two answers to this one:
1) It's considered good practice to keep your design (css) and your data (html) separate. By adding in-line styles, it makes it more difficult to revise the look of a site, it makes it more difficult for future programmers to modify your site, and is overall NOT the best way to go.
If everything is in a CSS file(s), then you can change the entire design of your site without having to mess with the data (HTML) of the site. This is ideal.
2) Yes, a lot of people still use inline styles very often when tweaking something small, regardless of "best practice".
