Less code, cleaner mark up, easier/quicker for search engines to index, lighter bandwidth usage, just to name a few.
Personally, I believe shifting to tabeless layouts allows more flexibility in implementing designs. And yes the cleaner code will prove to be a great advantage when updating the site in the future.
Not only is the code cleaner, but by moving the layout information into one file you can completely change the look and feel of your site through changes to one file. So the biggest reason for moving to CSS style sites is for maintainability in my opinion.
ESPN saved 2 Terabytes a day in bandwidth: http://www.mikeindustries.com/blog/archive/2003/06/espn-interview