Well it often breaks column and row scope in screen readers if you don't explicitly state them. I've had that more than once where using block level containers inside TD often "lose the association" to their parent TH on the row or in relation to TH inside the THEAD. (assuming you are even building the table PROPERLY). It can be fixed by stating either "headers=" on the TD or "scope=" on the associated TH, but why use extra markup if you don't have to?
But again, with most developers not even knowing that CAPTION, TBODY, THEAD, TFOOT, COLGROUP, COL or TH even exist (much less Mozilla still not even supporting COL properly some thirteen years after the bugzilla entry was made), much less the concept of the SCOPE, AXIS or HEADERS attributes it's probably why so many people think that tables are always broken in screen readers-- when in general they're just not using them right.
See how I feel the need to backhand someone every time I see:
<td colspan="8" class="header" align="center"><b><font size="+2">Table Title</font></b></td>
doing the job of
AS a rule of thumb I've taken to saying that if you end up using numbered heading tags, paragraphs or DIV inside your TD, (or LI) it ceases to be table data (or list items) with VERY rare exceptions