I wonder if it ever had a positive impact on sales. It seemed so overkill, even back then, and today even more so. The idea was nice but imagine the additional costs for hardware development and also for software. Seems so silly nowadays.
For games where the VMU wasn't useful, they usually just put a logo or a looping animation.
For games where it was useful, you got cool things like picking football plays on a private screen, or having a private screen to see your items, hidden from the other players in the room, or inventory/status information. Etc
I didn't use any of the VMU contained games though, coin cells are too expensive.
