We say that a stiff grab could maybe prevent tori from moving, but this is not interesting. An interesting grab is a grab that can move the person grabbed.
I agree the grab should be able to unbalance and move nage. Katatedori is like the hub connecting the energies of uke and nage, therefore the grab should not be stiff (clutching the fist and trying to squeeze the life out of your parner) but natural and using the whole hand. If done correctly one sticks to nage no matter how hard it tries to swat/push/pull you...the only answer left to him/her is to blend.