Negative priorty could actually be beneficial, if you have enough Hp to not be finished off in one hit. that is a decent point though.You must have missed the part where I said "make it -1 priority, like Perish Song, so that they can Taunt or Sub". What concern about Pain Split wouldn't that fix?
Maybe it's just me, but the "ooooo 999 Gengar learns it, how frightening" argument doesn't carry a whole lot of impact anymore.
Gengar is just my example though, one that people can identify with due to its popularity. There are plenty of other Pokemon that could use it well.
...Like Haunter. :p