The reason is to prevent the bacteria that causes rot from entering the iris. I plant them right away, but sometimes I lose a few. It's worth the risk to me, since when I'm in the mood to divide I don't want to wait.
Renee

Thanks, Renee. I feel the same way.

I've decided to keep them in the raised boxes for another year, and add the little babies I bought earlier this year still in small pots (the ones I shared a picture of in the shallow terracotta bowl). I'd like to create some Iris beds for them but don't have any ready at this time.