Myself, I wouldn't write it off completely, but I would certainly keep some walls up and we would never be all that close. But I find value in many levels of relationships, I don't feel the need to go "all in" with everyone to get something that benefits us both. I also wouldn't ever change any plans I had to make time for them, for example.
That's kind of how I've been, but I have been thinking about what would change this. As in, how long would we have this casual, cool relationship where they seemingly are fine with me being poly before I let those barriers down? How long until a Didn't Even Know becomes a fully fledged Noob? But perhaps the reason they were cool is because it was casual and when it becomes serious, they'll freak out.