Why sphinx? With fully configured sphinx search engine, your script can search on large amount of data very very quickly, like a speed of light compared with normal way.
On vBulletin 4, rebuilding search index takes very long time especially if you have lot of posts (more than one million) on your forum. My forum has about 1,5 million of posts. And to rebuild search index of about 10,000 posts, it can take hours. So to rebuild search index of 1,5 million posts, it takes days.
Because rebuilding search index was a painful process for my forum, i was thinking to use sphinx search engine for my vBulletin forum.
So first important point is : if your forum has lot of posts, you’d better to use Sphinx. But if your forum has small amount of posts, you do not need Sphinx search engine.