It's one thing for us to use it for our own selfish needs, but it's entirely different for others to profit off of it.
And who is talking about making a profit?
My kids love putting quarters in to play games. They can easily just open up the coin door to grab them out again, but putting them in is part of the experience.
What difference does it make if the quarter is put in before or after game selection?
None.
To your kids, none, you're exactly right, that's why we don't consider NOT supporting this feature to be a big issue, there is no legitimate reason to need it for your average user. Your kids know when they can insert coins because you told them and if they do it wrong, you can easily give them the coins back until they learn.
For a commercial on-location cab it's essential. Customers aren't your kids, with real customers they'll insert coins in the menu, and wonder why they don't transfer, they'll insert a coin in a game, quit the game, and wonder where their coin went. As I've said, the development team don't want to see MAME being used in this way, so by not supporting such features, and discouraging discussion of such features the ability for people to use the software in this way is limited.
Your kids don't care, and at the same time it prevents abuse of the software in a commerical environment. It's a win for you (and your kids), and it's a win for the development team. The only losers are the ones looking to try and abuse the software for profit.
Arcades are struggling, and people replacing legitimate arcade cabinets with illegal multi-game cabinets isn't helping the cause. Mamedev want *nothing* to do with that so discourage features which make it possible.