I don't think that's what's happening there....in "A Scandal in Belgravia" Mycroft says that "Of course [he doesn't trust their Secret Service], they spy on people for money." I think it was a test, which John passed by refusing to take money to spy on Sherlock.
I think we can assume that Sherlock doesn't truly financially need a roommate, he needs a "minder". I wonder how many other potential roommates Mycroft ran off after proving they didn't really have Sherlock's best interests at heart? :)
And the real reason I dropped in this thread: Congratulations to "The Abominable Bride" on its Emmy Win for TV Movie!