Was there a case before where whether we actually played BoM mattered?
Yes, with Conspirator, as has been mentioned before.
My understanding is that in the brief moment between you choosing to play BoM and BoM actually being in play, BoM is both itself and the card from the supply you chose. Bom says "Play this [...]", so you are playing BoM, there is no "would" wording involved. Then once it's on your play area, it's only the card you chose, until it leaves your play area.
BoM's instructions are not like any other Action card's instructions. All other cards' instructions are triggered when the card hits the table and is in play, after any when-play effects from other cards (like Moat's reaction or a previously played Urchin). So it's always like this:
1) Choose to play an Action card.
2) Put it in play.
3) Resolve when-play effects from other cards.
4) Resolve the card's instructions.
(There is an exception if the card can't be put into play, because it's in the Trash for example. Then step 2 fails, but we still do the other steps.)
If BoM worked like this (which it doesn't), it would be like this:
1) Choose to play BoM.
2) Put it in play as BoM.
3) Resolve when-play effects from other cards.
4) Resolve the card's instructions:
4A) Choose a card from Supply costing less: Choose Sea Hag.
4B) Put BoM-as-Sea Hag in play. (This fails since the card is already in play.)
4C) Resolve when-play effects from other cards.
4D) Resolve BoM-as-Sea Hag's instructions.
We know it doesn't work like this. It would mean two cards are played, BoM-as-BoM and BoM-as-Sea Hag. Conspirator would see two played cards. The only way BoM can work is if it's a before-play (or a when-would-play) ability, just like Trader is a when-would-gain ability.
1) Choose to play BoM.
2) The when-would-play triggers: Choose a card from Supply costing less: Choose Sea Hag.
3) Put BoM-as-Sea Hag in play.
4) Resolve when-play effects from other cards.
5) Resolve Sea Hag's instructions.
So a BoM is never played. It's in step 4 that we check for tokens on piles.
Again, see here:
http://forum.dominionstrategy.com/index.php?topic=12906.msg482494#msg482494