Personally, I'm tickled pink every time someone brings back an old, dead thread. If that didn't happen, I wouldn't get to experience the predictable joy of reading wisebeardman's "gravedigger" posts, nor would the rest of us have much to complain about...
*grumble grumble...stupid zombie threads...grumble...blah blah some other complaints...grumble*
Okay, so seriously, it makes sense to do a search on a topic if a user is looking to have a specific question answered- they can search, see if they find their answer, and be done with it (in theory). If they don't find their answer, they can start a new thread. No biggie.
Obviously that doesn't work that well...but it's a good theory...