[TECK]

Personally, I think it is important to have accurate results. If deleted or edited posts still show in search results (when it should not) as well that once a day you perform a full reindex (when you should not), that affects the overall search accuracy as well the server performance. Think of it this way: you have several threads in a forum where the users change a price for their "to be sold" items published few days ago. Because your search index is not updated right away (10min max), other forum users will never know that the price on certain items was revised until next day when indexes are rebuilt entirely...

A better example, related to search accuracy. Let's presume you use the default vBulletin search, query the entire posts for 'spaghetti' (most intensive) and display the results as threads. Then, you perform the same type of search with the vB.org Sphinx search. You will notice the number of missing results very easy, while performing the search with vB.org Sphinx product. There are many other aspects that I rather not cover, because it will sound like I'm trying to advertise my product... Fell free to ask more questions in my forum.