If the uke does not follow the technique, neither partner is learning anything... and injuries can occur. Also, nage can try extending uke more, which will cause them to lose balance, and thus, they will be unable to spin out. Shihonage, like any other aikido technique, is simply a method of training to realize the principles of aikido, if uke cannot follow, explain why they should... and if they still don't cooperate, ask them to leave the class.
I think extending and unbalancing uke (kuzushi) is needed in every technique. So it is in shiho nage.
But: Is following nage a concept of your aikido also on an advanced level? Or are you talking about beginners only?