This site gives some pretty good reasons if you don't mind reading through it: http://webdesign.about.com/od/layout/a/aa111102a.htm
. It's really just not a good styling practice. Tables weren't meant to be used to style pages; you can style a page properly and the same as you would with tables with CSS and without all the messy table tags in your HTML. Also, you could run into a lot of problems with tables, especially if you have overlapping code (i.e. if you need a whole column to be connect, for example in a form, you run the problem of having to include all the other columns, since the cells are lined up like a book, left to right and not directly up and down).