
They confuse search engines (mildly, admittedly), require shitloads of code to do a very simple job, and require you to edit every page of a site if you want to change the layout.
There's more, but I can't be arsed. Tables should be used for displaying tabulated data, and not for laying out web pages.
This has been a public service announcement on behalf of the W3C
( , Fri 10 Sep 2004, 14:01, archived)

to be frank, if you desinged the site properly in the first place, you wouldn't need to change the layout.
( , Fri 10 Sep 2004, 14:09, archived)

and over 5-6 years you never ever want to change anything about the layout? Unlikely. What if you have a new product or service you want to add? Or a new dynamic feature? If it's in tables you probably have to scrap the lot.
Plus, all the format information has to load with every page instead of just cacheing from a CSS file once on your first hit.
( , Fri 10 Sep 2004, 14:13, archived)