Currently there is a limitation for one user to access multiple products owned by others because they are added as a contact for the client then that email address cannot be used with any other account.
What I would like to see is this:
Client A - Owns hosting product for foo.com
Client B - Owns hosting product for bar.com
Client C - Owns hosting product for baz.com
Client C is also the developer for foo.com and bar.com. When client C is logged in, they see under their Products/Services baz.com. But there is another option for Additional Products/Services where he sees foo.com and bar.com hosting accounts and can login to cPanel, etc.
This Additional Products/Services would also be a tab in the admin under the Client Profile.