Auburngirl-
I think you've got it right, I relate this (on a smaller scale) to someone who wants to quit smoking. I've dated smokers before even though I've never smoked and don't care for it. They always say they want to quit, and I'll be there for her, but if she's quitting for me then she's quitting for the wrong reason, and if I'm only in the relationship because I want to help her quit, then I'm in it for the wrong reason. They have to want to quit for themselves, whether I'm around or not.
This is not to say that someone with PTSD has to "cure" themselves, but the reference is that they have to want to heal for themself, and not for thier partner's sake. Likewise, if someone is going get into a relationship to be a carer in the first place then I think that's a bad idea (JMHO).
With my girlfriend, I met her through a personal ad that she answered. She had many traits that I was looking for in a woman, and when we met we hit it off right away. I fell in love with her not knowing about PTSD, and thought her issues were because of something else (that was understandable as well). But I was already hooked and fell for who she is on the inside and for what she wants from a relationship (didn't hurt that she's a hottie, either!).
She told me that she has these issues and gave me the ultimatum of being with her and putting up with it, or realizing that it may be too much to deal with. I stayed, not because I want to care for her, but because I do care for her. Additionally, the qualities I find in her with regards to a relationship are qualities I don't know if I'll find again...I've been single for a long time looking for someone like her, and I only see her PTSD as a challenge, not an obstacle. I hope that makes sense.
I do hope that at some point she does share more with me, and I think she's on the right track as she has opened up to me quite a bit. I feel the distinction here is just as it is in any 'normal' relationship: you have to want to take care of the person because you love them...not because you think they need it. And that goes both ways, being a carer is not the same as being a caretaker, I think there's a big difference between the two. But that's just me thinking out loud....