I have seen about two neo-Caths sign up since I joined. Neither of them left because of sedevacantism, nor was sedevacantism even discussed with them. As far as why I don't want them here, they are arrogant snobs who hate Tradition.
Stevus, arguing with neo-Caths eventually becomes pointless. As Pius X said, "It is extremely rare to convert a modernist". To the modernists, 2+2 can mean not only 4, but 10, 100, even 1000. To them, the possibilities are endless, and the rules are meaningless. That's why you are wasting your time arguing with them at CAF. They have a spiritual veil over their head that only God can lift. You might get better results debating with them on a forum that isn't modernist since they'd be outnumbered, but Matthew doesn't allow them here, nor does anyone else want them here anyway. I would leave CatholicInfo if they were allowed on here, and I'm sure many others would also leave. I wouldn't mind FSSP Trads joining, though. As long as they weren't anti-SSPX.