So, you're in a preliminary conversation with someone, with an eye towards the possibility of dating. They seem like a lovely person! Smart, sexy, articulate, numerous common interests, similar goals. As the discussion goes forward, you share a bit about your current and past relationships. They do the same, and suddenly, things take a wrong turn at Albuquerque!
Turns out that they have 16 examples of past relationships where the other person was a raving bitch, completely psycho, totally unreasonable, or didn't communicate well with them.
Why is this a problem, you may ask? After all, you're looking at the possibility of dating this person, and YOU aren't at all like those past partners, right? Hahahahahahaha! What's the common thread in all those relationships? The person you're looking at dating.
Let's suppose, for the sake of argument, that your relationship with this new person will, in fact, be different than all the other relationships they've had in the past, you being such a special snowflake and all... Why is this still a problem? Because you two are poly, and their partner selection sucks eggs, which means that you're going to have metamours that will have very unhealthy relationships with your beloved partner. Thanks. I'll pass!