On the other hand, if you are a midsize or large business, with minimal effort you can effectively own a vendor that will act as an unofficial extra department.

On the other hand, if you are a midsize or large business, with minimal effort you can effectively own a vendor that will act as an unofficial extra department. You give them enough business to become their biggest customer and they are effectively hooked for life, because who's ever going to drop a client providing 40-50% of their total revenue?