I think the opening sequence with her and Marco was evidence of her vulnerability. Joan Didion is very smart with her writing in this film and no-so-subtly hinted that Marco was a selfish jerk who did not treat her well. The way he reacted to her abortion and then "split," as Bobby stated, without a second thought made that pretty clear.
So while Bobby may have been a "step down" from Marco - who had his life together, for the most part, a great apartment and a moderately normal lifestyle as a hippie artist - he showed her affection when she was alone, came to visit her in the hospital, and obviously had a dangerous, romantic streak that made him attractive and mysterious. Eventually he turns out to be emotionally manipulative and rather abusive but by then she was in love with him and even more vulnerable because of it. I think many women - and many men - can relate to this easily.
I think their love story is extremely well written and so on point to the experience of being a junkie. They pull each other down and bring each other back up but in the end it's an endless cycle that mirrors their addiction.
I'm optimistic and a hopeless romantic at times so I hope that the end shows them taking on the world and their addiction together, bravely, but I know too many addicts to know that their love intertwined with their drug usage will only bring them down.

I've been in a eerily similar relationship. After I graduated college with a BS in Biochemistry, I thought I'd take a year off & save up some money and apply for graduate school the following year. Instead, I got bored really quickly and while I was partying with a few friends, this guy (now my ex) asked if I'd wanna experience the best kinda high so I let him shoot me up. From there, we isolated ourselves not only from friends but I was told not to be in contact with my mom.
Yes, this all sounds crazy, especially with such a well-rounded person as myself but I have always had underlying mental illness i.e. depression and major anxiety disorder; I was self-medicating. I've been clean since I saw my ex leave for prison in Feb. '12. Turns out he's a diagnosed sociopath and I was only 1 of his countless "usees".

That she's attracted to a "real loser" as you say doesn't speak much for her.

It started in the mid to late 60's. Kids ran away from home to find something else. Parents were strict and what they said and did was the law. The baby boomers as teens and young adults chose a different path from the conservative households they were raised in. Kids took off and hitchhiked around the country. College attendence skyrocketed (partly due to Vietnam). Hell, look at the Manson Family, those kids were mostly all runaways. For some, going home was total weakness and a sign of defeat. For others, it just simply wasn't an option. Jim Morrison is another example. His folks weren't at all supportive of his choices/career/lifestyle. When he was asked about them, he told reporters they were deceased.

She does tell Bobby that she has a mother and a father and a little brother and a lawn. And that she went to art school. The early 70's were still a little bit "rebel without a cause", and I see her pursuing an artsy life in NY to get away from a stifling Midwest existence. A woman would have to be some kind of nutty to go from Marco to Bobby and see that as a step up.
OrShe could have been stolen in infancy by pirates. Who knows?