I would say very simply that a tsuba makes you lazy. Learning how to protect your hands and knuckles etc. is all part of the training.
It would be equally accurate (or inaccurate) to say that *not* having a tsuba causes your partners to pull their strikes to avoid injuring you. Learning how to deal with realistic strikes is part of the training, too.
I once had a partner hit my (wooden) tsuba hard enough to split it in half. I'm pretty happy that the tsuba took the blow instead of my hand.
I've also seen quite a few techniques where the tsuba plays an important role. Certainly they are considered essential equipment in both my current and previous dojos.
So I'd say a blanket statement like yours says more about you than about the value (or not) of a tsuba.