The advantage of using tables for framing in tabs and buttons in particular is the "shrink wrapping" effect that the table has on the content. A table is the easiest way to make an element size to its content in browsers that don't support "display:table" or "display:inline-block" (IE6/7 strict mode, and IE6-9 quirks mode). If we used divs for framing buttons we'd have to fight with "inline-block" emulation in these older versions of IE, which presents its own set of problems and side effects, and does not provide any noticeable performance gain.
In summary, our thought process on deciding whether we need to use a table usually goes something like this:
a. If we need feature "x" in the framework (e.g. framing, or shrink wrapping layouts)
b. AND if there's only 2 ways to achieve feature "x" in old IE browsers - use a table, or perform expensive JS calculations.