There is a discussion on the forums: http://forums.movabletype.org/2009/02/can-we-search-both-tags-and-entry-texts-in-the-same-result-set.html that speaks to the fact that the search code retrieves entry text search results or tag search results but doesn't work with a combined search. Sometimes users will tag content with words that don't appear in the blog text (such as a name) and when they do a standard search it doesn't return any results.
Reported on Movable Type 4.2
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