There was a time during a tournament I was judging, where I had asked a player to substitute a translucent die (not sure if I can say "regulation" since it's not an official Pokemon die, just one that meets the standards) for their official coin. The reason I requested this substitution was because the player was obviously having trouble flipping the coin; the coin often got tossed off the table, and one or both players would stop playing to go look for it (on dark carpet in a somewhat cramped space, making it difficult to find). This was happening so often (the player didn't seem too careful about trying to keep the coin on the table) that I was afraid it would affect the pace of the game, or give the player an advantage through stalling, so I asked that the player use a die instead of a coin. I'm working on getting a small stock of tournament-standard dice for our kit in case we have to do this again, since the die I brought over was from our lost-and-found.
That would be the only instance where I think there'd be a good reason for a Judge prohibiting coins, if it were too much of a hassle for coins to be used (hard to retrieve if they fell off the table, majority of attendees known for not being good with coins, stall issues, etc.).
