Why don't you use the MT Searches plugin for this?
I recently implemented this (in a multi-blog environment) and it works great. See it in action at http://www.stanthecaddy.com/search.html. (Note that in this implementation I also limit the results to a single category).
I guess that's a good reason - depending on the intent and frequency of rebuilds. It works well for me on the Seinfeld Blog because rebuilds are very frequent, due to the MT-powered forum on the site. :-)