Primarily because of backup reason.
Some of my apps, especially the largest one, don't necessarily need the vB environment and would do just as well as "standalones".
Right now, when I want those tables backed up and/or transfered to a test server I just blindly mysqldump the whole secondary database. Same with restoring.
After merging I'd need to specifically pick or exclude tables (forum takes about 500MB, 2nd db only 20-30MB). Knowing myself, this will end in confusion
Would switching between databases cause any kind of trouble with the forum?
Like ... my script switches to db2, forum script tries to access its own tables and ... *bam*?