I do drills all the time with my dive partner, because doing a skill a few times in an OW class is not keeping sharp.
BUT I think that there is more than enough to learn in an OW class without adding a task that will not be used in 2006. It is standard operating procedure to have an octopus/alternate air source. PADI spends a lot of time on the correct positioning of the alternate air source (within the triange of the shoulders and the chest), reviewing it as part of the predive check, etc. Going over it with the buddy,etc. This is all covered in BWRAF. Its why they are bright yellow . YOU CAN"T MISS THEM.
So why in the world do we have to pretend that we don't have alternate air sources ??
That said, practicing donating a primary is a good idea, as there is some likelihood that it could be a scenerio. This does get the diver used to taking the reg out, handing it over and taking the octo/alt reg, etc into their mouth.
Whoever has the air, donates a reg. Period.
This is a good skill to practice. It can be almost automatic.