I share Photopost as well.. but since Photopost is an addition, I prefix those tables not vBulletins.

Same here, no prefixes on normal vB stuff~ :)

I don't know photopost - is it vB hack or addon ?

I mean completely different applications - I don't mean addon's to vB like Articles, Arcade, or even Flashchat - these are part of your vB and should use the same db (and should use table names that don't clash with the standard vb ones). Prefixes are then obsolete as the DB has a unique name, and all the tables within it are unique. Nothing is gained.

I'm sure people use them simply because they can - but I can't think of any good reason you should need to use them, and so far no one else seems to have.

From what I know... photopost doesn't have to be used with vB, though it includes a lot of stuff that will do the integrating etc.

We usually have one seperate db for each site on the server.

I haven't run into a single hack that didn't use them, I don't go around installing random hacks though...

Nor do I!
I do have a parallel forum (testbed) running on a phpDev setup, and experiment locally any mods/hacks that look juicy, but I only ever install mods onto my "live" forums that are either useful, or requested/required.

What hacks are you talking about btw ?