Tell us if you decide to talk to her and how it goes. If you don't want someone to have a key to your apartment, that is OK!
-Michelle

DZTUBAGIRL - While she is good at paying bills on time and generally does not spend the night there, that gives her no right to compromise your safety by leaving the door unlocked so her friends can go chill out whenever they feel like it. What if someone other than her friends comes by and either A) steals property from both of you, or B) comes inside with a bad intent and does something terrible?

I had a similar living situation. It was supposed to be four women in a house, but it turned into three women and a couple. It was extremely awkward and uncomfortable. Here is how it was resolved. We spoke to her about him living there numerous times, but he continued to stay. He was a nice enough man, but it was stressful since we hadn't expected this living arrangement. One weekend when they were away, her mother called and another housemate told her EVERYTHING! They moved out soon after.

So I think that you should talk to your roommate. If she doesn't stop this situation, tell her mother. :p Or maybe not.