Tables were intended for tabular data. Using them for layouts was just a hack until css and layers came along. IIRC, they render more slowly than layers because the browser needs the entire table before it can determine the layout. In any case, once you start getting into complex tables nested in tables, the code becomes a nightmare... and the vertical alignment in data cells is usually defaulted to center meaning you have to use 'spacer' cells just to make things look right.

...it's been aeons, though, since i've even dealt with table layouts so for all I know none of what I said is still true for modern browsers, but I still think they're not a good idea.