Search the category names for what though?
I mean the search script returns results of ads that match the search parameter. What would matching a category name do. If you are searching for say the word blue to return ads with that search parameter what would you expect if a category name has blue in it.
Just because a category name has blue in it does not mean the ads within the category name contain that search parameter so really searching the category name has nothing to do with the results or even the search parameter.