I think there needs to be a range of practice at all levels: practice where nage knows what attack is coming; practice where nage doesn't know what attack is coming (either randori or giving uke a choice of maybe two attacks); practice where uke knows what defence is coming; and practice where uke doesn't know what defence is coming.
I don't want the dojo to become a street brawl, yet what we learn must be usable on the street, or we are developing dangerous responses which will get students hurt if they get involved in a street situation. This isn't about going round looking for a fight, but about being attacked, or stepping in if someone else is being attacked.
So we must be able to defend against a roundhouse punch, not just yokomen uchi; against a punch that is pulled back or in combination rather than a single tsuki that stays out. We must be able to develop reflexes if someone attacks unexpectedly, so we do some of that (not just for students: last week I was practicing with two attackers, and a third decided to join in from behind me!).