The main event will be entertaining but won't really answer many questions. Rumble has said in the past, he isn't sticking around at HW too long. So this fight at HW is for the sake of fighting at HW and against a named opponent.
Arlovski is esentially fighter an opponent who has so place being at that weight, if he wins it's against a bloated MW/LHW, if he loses he loses to a bloated MW/LHW.
Makes no sense and is an unnecessary risk, but will be awesome to watch while it lasts!