In my 41 years as a Carpenter and 31 years of that time as a Licensed Builder, I've grown really tired of this silly argument. It's like arguing the benefits of not boiling water twice in case it removes all of the goodness …..  ??

There was a time that decking was never reeded. In 1983, The "Timber promotion Council Display Centre" in Blackburn, Victoria, began demonstrating "Reeded Pine Decking" as a less slippery, better drained surface on stairs and decks with the alternative choice of also laying it smooth side up.  All of the models on display initially were shown with the grooves (reeded side) up.
Innovative builders quickly began using this nice looking, cheaper than Merbau, grooved decking as a sales edge. Sometimes there were patterns created by varying the ups and downs.. ie: 4 up and 4 down, or a very handsome looking smooth deck with a reeded edge margain. Many civil engineers specified, for their own good reasons, ribs up on commercial decks, balconies, beer gardens etc.
Carpenters quickly discovered how hard and painful it was kneeling on the ribs to nail large areas reeded side up and some wonderful <moderator: delete language> stories were invented to convince customers to accept smooth side up.

It is foolish to think that facing the reeded side down to allow airflow actually aids drying. What really happens is "capillary action" which draws in and stores more water, possibly contributing to swelling and rotting.
If the job is done properly, the joists are first protected by covering their tops with suitable flashing that drops rain water clear of underlying framing timber. In nearly all cases of rot in decks, it is the unprotected joists below that suffer first and the rot spreads to the underside of the decking later..

Tradeschool teachers who preach to their students after watching backyard garden shows need to get in the real world.

It is purely a matter of customer choice, based on the relevant criteria available.