The problem of people thinking that they might get away without a permaban and then arguing forever if they get permabanned. If people know that they're going to be gone for life when they get banned, then they don't do things to get banned. It's much like the old tougher penalties for certain crimes. The tougher penalties are put in place so people don't mess around at all.
The problem of the many temp-banned people picking up grudges against Staff in general (or certain individuals amongst Staff) after they've come back.