I think I would like the 2 pref idea better if it went 18-13-9-5-2. But I am not the one who has to sit through membership selection until the rooster crows so I will shut up now.
It would be interesting, though, to see if the rushees refusing to put a 3rd option were all doing it for the same sorority - i.e. if there was one sorority that had to keep inviting everyone back, so the rushees had to keep ranking it - or if it was a variety of sororities, and they just had trouble forming a liking for more than two.