>>I think, orangeskittles, what you said about the "reminding him of April" works, especially in the show, more so than the movie. I mean, Mark even says that Roger found the note from April, after she had already killed herself (at least that's my interpretation). So Roger probably blames himself for not being there for her, hence all of his protectiveness over Mimi with the drug dealer.
Plus...he obviously likes her despite not knowing what to do initially, and despite her habit. Any good boyfriend is protective of his girl, especially if she has problems that could kill her!
"There is no use trying," said Alice; "one can't believe impossible things." "I dare say you haven't had the practice," said the Queen. "When I was your age, I always did it for half an hour a day. Why, sometimes I've believed as many as six impossible things before breakfast." --Alice in Wonderland