Fair question. Semantically the <table> tag is created for just that, tabular data. If you use it for layout there are a number of problems. First, it is harder for screen-readers and other accessibility-based tools to decipher a website laid out with tables than one properly laid out using standards-based development. Second, you are adding needless bloat and confusion to your markup, as you will have completely unnecessary tags littered throughout your HTML. Third, you will be going against standards-based development, which is a worthy goal in and of itself.

CSS exists to separate markup from style information, that's its entire purpose. To use tables to lay out a page is to explicitly include style information in a place designated only for semantic markup.