Depends - is anything else using that database besides the forum?
If no, then not really. All I can see the prefixes being nice for are people who have 1 database - you'd want prefixes on the tables so any other php/mysql stuff running on that database don't end up with conflicting table names (such as a front-page having the table user, and the forum having the table user - it can be done, but it'd be nasty if you installed a forum over that table).
If yes, you might wanna try and fix it unless you're damn sure nothing else on that database will ever use a table that has the same name as the forum - CMS's and Forums don't play nice together for instance.
A way around that though is if you install something later, make sure to give it a prefix. That way the forum could have a table labeled "user," and your CMS might have the table "joomla_user"