Yeah, screen readers seem pretty useless if they cant handle a web page with tables-for-layout, since these are quite common whether we like it or not.

A sensible heuristic would be to treat tables without TH or THEAD elements as layout-tables, ie. just read them serially, while treating tables which contain at least one TH-element as a true semantic table. I'll bet this is the correct interpretation 9/10 times.