For every wiki that is created here on wikihost.org, a new copy of a plain basic wiki is made.
This copy holds everything, the wiki needs to run, in one directory - this includes the database structure.
This way, it is absolutely impossible to have "database accidents" - for example, accidentally switching contents between different wikis.

Another advantage of having seperated wikis is that it only takes a second or two
to zip all files belonging to one wiki into an archive for download.
If you feel you need to move your wiki away from wikihost.org - just do so, it will take you only some seconds.
For information on _how_ to backup up all the files in a second or two, see HowTo:Backup Your Wiki.

Other side of the medal:
Some might feel that not having one big user database for all wikis is a disadvantage. They may be right...

At last, there is a seach/scan about all wikies.
A inrawiki link like roWiki.start would be helpfull.