I remember that Nishio sensei was not that fond of morotedori, because uke's both hands are occupied while tori has one hand free - not a very wise attack form.
I teach my students to do morotedori so that they grab the arm and immediately move behind tori, into sort of a yonkyo position. I think that I learned it from Ichimura sensei, who was a student of Nishio sensei.
Both ways of dong morotedori are interesting, of course. They lead to quite different aikido technique solutions.