Hi Janet, I am thinking of sword attack, which is what Aikido is supposed to be based on (not parking my car). If the irimi is good, the tenkan will kill you. That's the way I see it. I know we all usually start Aikido from avoidance, take balance, make technique, etc. etc. but as you hone your skill you need less and less avoidance until you can just walk straight through your uke regardless.
With that in mind, I personally think the two irimi and two tenkan practice method of most dojos to be a huge mistake. Or perhaps, it only makes sense once you realise it to be a mistake.
I would say when is it useful in Sword rather than it will get you killed.
Using irimi empty hands against a shomen type cut is an example. If you irmi and tenkan then your hand drops down on the attackers wrist behind their hold of the sword or even onto the handle if you are good enough.
If you raise your sword in alignment with the attacker and enter, like a mirror image, like ikkyo with the sword, and enter irimi and tenkan then you will find yourself facing the same way as the attacker but your sword is now ready to cut down his back or even arms if you wish or shoulder take your pick depending how deep your irimi was.
Irimi tenkan is a matter of when rather than doesn't work.