Originally Posted by k0nr4d

You should be using a separate db per site for several reasons:
- Table name overlap (you might need a users table on each site)
- Security (sql injection on one site will limit damage to the one site and not all your sites).
- Portability (you can move a single site to another server if you need)
- Corruption (single database getting corrupted can cause complete loss for all your sites)

I can't really think of a reason to use a single db like what you are describing.