As an example why can I not search forum posts for an exact phrase like you can in the help section?
Or do a search within quotes "..." ?
Or a boolean search eg using AND OR etc ?
I've found that many of the forums I use that have a large number of users with frequent posts, have a search function that produces less than desirable results. However, if you are patient and re-try your search and get a little clever with your keywords, you can get some decent results. And yes, always use the advanced search.
I suppose they could benefit from using google, yahoo or other highpower search engine, but then we'd probably have to put up with advertising.