More than once I have been wanting to look up someone to talk about or offer an item/niche/whatever they specifically requested in their signature.  When I don't remember the username, I have found that searching the forum doesn't yield the results for keywords in signatures. 

My suggestion is to add a search function on the directory page that lets you search only within signatures, same as you would normally do a forum-wide search.  Or add it as a "category to search" in the section at the bottom of the advanced search page of the forum search.