Great... I find the thread after everyone is pissed off.
My opinion. SU needed a place to live. She wasn't interested in romance or a relationship.
I suspect she was moved out of the other house because she had overstepped boundaries there or overstayed her welcome, or just used them too much. (whether it was poly or family or friendship...)
She's in survival mode... desperate to survive. You were her lifeboat for a bit.. but she's also used enough people to make sure she has a plan B.
Perhaps she's learned, more likely she's burned out her next link in the chain of relationships. She just needs a place to stay, but she doesn't have the resources, (Job, bank account, car, etc.) to qualify for a relationship. She managed to stay afloat for several months at your place. Consider it a gift to the universe, gathering good karma, whatever..
Every relationship is different. Even bad experiences and failures teach lessons.
I'll save the comments about legal relationships vs. ethical relationships for later.
Josh-- We've seen a wide range of attitudes and personalities...It's easy for us to figure out what is wrong with someone else's relationship... But you are living your life.... Do what works for you.
I'm a unicorn... and dating a couple. My opinions are based on my experiences...