I think the idea of having multiple sweethearts and not trying to detract from sorority numbers is just fabulous. I wouldn't wish harm on them, so why not just work together? (well, where you can) I will say the idea of fraternity cling-ons still just makes me a little ill, but that's me.
I can say that my chapter has gotten a number of new members because of our relationships with fraternity guys. To hang out there, on an anti-greek campus, it means you have some level of acceptance of the system.