[Andreas]

Well, in vBulletin the user table holds

• User ID
• Username
• Password
• Primary Usergroup
• Secondary Usergroups
• eMail-Address and Parent eMail-Address (if COPPA-User)
• Style being used
• Homapage URL
• ICQ, AIM, MSN, Yahoo IDs
• User Titel
• Join Date
• Last Activity
• Date/Time of last post
• Number of Posts
• Reputation Level
• Various settings
• ID of Avatar being used (if not custom)
• Flag for showing "You've got a new PM Popup"
• Language being used
• Number of total and unread PMs
• Birthday

Now, if this table is shared between various installation with the methode posted above (which ist of course possible) there will be a lot of problems.
For example the post counter:
You make 1 post in board 1, but as board 2 uses the same table it will also show 1 post .

Or the Style. the User might chose to select style 1 on board 1 and style 2 on board 2 - this isn't possible.

And so on and so on ...

I've just take a look into phpBB user table, and found out that it holds similar information.
So similar problems will occur.