Wrist grabs. Sure its not a great attack. But if done for real I think it will be a lot more effective than "grab and push to center" or "grab and immobilize". One seminar instructor said something to the effect, that morotedori should be applied as yonkyo.
Even in ikkyo, one hand is "grabbing" uke's wrist or hand while the other works his elbow. In pretty much every aikido "hand" technique, we "hold" uke's arm in some way.
But in aikido strategy, it's always after uke grabs for us (or punches, or whatever).
What you describe in the OP could just as well be the beginning of ikkyo, nikkyo, sankyo, yonkyo or a number of other standard techniques instead of just "slinging" the attacker by his arm. But generally, it begins when uke extends his arm toward us.
Best to you.