I voted somewhat important. If it's a question of ukes attacking full force, I think training safety should be a prime consideration, outweighing all others. Yeah, O Sensei had people come at him with live swords, but that was after he'd had one heck of a life and had developed a lot of skill. The goal should be to learn Aikido safely, not see who's tougher.
Besides which, attacks can require as much study and practice as any throw or control, so teaching good offensive structures means more work, and I'm too lazy.