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.
Rupert, thank you for your reply. I have also heard it said that aikido is meant to deal with multiple attackers or to be able to take down someone with least harm (the latter say for a drunk fool or out if control large child or if you just don't feel like putting your attacker through a window or into a beam) and hence my examples of choosing, following direct irimi, to tenkan and bring my attacker along a different vector. Options are good.
I also find, personally, due to some combination being short and very hippy and maybe some psychological factor I'm not aware of that for me there are simply times, once I have uke off balanced and controlled, it just feels right for us to turn.