a few points..
– the less non content related stuff in your markup the better. you want a low code to content ratio.
– css is used to separate presentation from content (rather than putting all your style code in each page of your site). it is a web standard and as such, if you want to develop websites and expect users/bots/search engines to interact with your sites they should be compliant to standards (or published grammars) such as xhtml, html, html transitional and so on.
– Using an external stylesheet means you can make global changes on your site by editing usually just 1 css file.. it also means that your page size is smaller (due to not having to embed css or specify it for diff elements) and your page has more spiderable content and less kludge.
There is no excuse for using Tables for layout in this day and age imo.. there are literally thosands of free 100% CSS layouts (and tutorials) .. the most common being 2 column or 3 column css layouts.. Google is your friend.
If you are interested in Optimising, then your site should be optimal in every way possible.. thus CSS is not a choice, it’s a must.