There are a lot of table vs. CSS debates.
Some of what I see as the advantages to CSS are:
1) Generally MUCH smaller code. I can generally shrink each page by 30% or more depending on how well the page was originally coded by switching to CSS-P based design. This not only speeds up page loading considerably, but gives you a better text/code ratio
2) Better flow of text being presented to the engines. Remember regardless of how it appears on the screen, it's what the engines see. So if your text is broken up by different cells, then the engines may not be reading it as you intend.
3) You can use CSS-P to place your more important text at the top of the code and still display it wherever you want it on the page (i.e. below graphics, nav menu's etc.)
There are other advantages, but the above three are what I see as the biggest SEO wise.