Pros/Cons of using multiple databases vs using a single database
None of performance, stability, optimization are true. Does anyone have a solid argument or reference article why these would be true?

Resources are not allocated to a database: the SQL Server Instance balances resources so it makes no difference

You lose:


data integrity
restore integrity (data in DB7 will be later then DB1)


You gain complexity:


security (users, roles etc) have to be in all databases
you'll have some data that doesn't fit into 1 database nicely


Options:


splitting a database onto separate disks can be done with filegroups
use schemas to logically separate data (based on other answer)