If someone grabs my shoulders and stands there strongly, I can't break someone's nose? If someone charges at me like a bull, I can't just simply throw them right into the shomen? What about a sharp open-handed backhand to the eyes?
You can randori for a bunch of reasons. The rules can change according to the experience of the nage, as Anne and others have mentioned.
The line between aikido and jujitsu blurs when you start to look at the combat effectiveness of aikido. Randori shows up aiki-flaws pretty quickly! The challenge is to give your uke their body back in the way they originally had it. I think one way to distinguish jujitsu from aikido is the condition of uke after the technique's finished.
At the Uni of West Australia Aikido, as we get more senior there are less rules in our randori but the rules are still there.
For example, as the speed and power of the attacks goes up we limit the type of attacks. And we begin to use gloves for punching and leg pads for kicking - mouthguards are handy too. When we're comfortable we'll open up the number and style of attacks, and increase the number of attackers.
Jun's later comment about the value of randori in certain (but not all!)schools of Aikikai are quite justified. I've seen a heap of "dancing on the mats" with little if any relation to a real situation. But hey if a club's not interested in effectiveness then it's not an issue. It's only an issue if they claim their aikido is effective self-defense.
I find that the more I test the limits of my technique through randori the slower and more sensitive my normal training is - speed and timing are things I've already worked on, so I can look at balance/linking/sticky hands and other such things.
[Edited by liam on July 11, 2000 at 09:34pm]