The tt, i, b, big, and small tags are not deprecated, but it is possible to achieve richer effect with CSS. What is the merit of using these?
Although yes, it's shorter and there's ease of use, the issue at hand is semantic markup -- basically, there's no question about what <strong> and <em> are supposed to mean to the user. This is especially important for cases where color and font-weight are meaningless (eg, to a blind person who's using a text-to-speach system).
If you're working for a US government agency (any government in the US, not just feds), you're bound by the American with Disabilities Act. Feds are specifically bound by Section 508 ... but in practice, it's also good to follow the Web Content Accessibility Guidelines. Companies would be wise to also follow the guidelines to reduce the possibility of a lawsuit and then having to redesign the side to correctly handle these issues.
ps. The one element that you didn't mention that's always bugged me is specifically <th> ... why the hell do people like using <td><b> ... </b></td> or <td class='header'> ...?