Take a look at the RSS URL generation system of MyBB(oard):

http://community.mybboard.net/misc.php?action=syndication

Since many RSS users are sophisticated users they ofter want only to track certain forums.

Yes, this is a very nice feature.

I agree that IPB should have this, or something of this nature. This is a very nice feature, and could certainly be implemented into IPB to enhance the user relationship with the RSS feeder by providing for a more detailed experience, such as the RSS Self-Service URL as MyBB offers.