It seems probus is outnumbered. It also seems that using the hyphen is definately not incorrect.
Using the hyphen is definitely correct.
The debate is whether *not* using it could also be correct. It is not, in my opinion. It is, in his opinion.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.