I'm particularry intrigued that this is the reason GF added Mark feature for competition eligibilty and as for recently, HOME altering the movesets for every 'mon entering your crib; Sejun Park's Magmar in VGC 2013 knows Follow Me, which would be very hard to obtain for average player and I think GF think it's unfair.
Ultimately, the entire premise of "generational lock" and then "battle ready marks" that wipe off the moveset was basically to not give unfair advantage to people able to obtain time limited Pokemon.
Them adding Ability Patches last generation was also probably in that direction, because certain legendary HAs have also been event-locked, however these have been legal over the years (last I can think was Multiscale Lugia in gen 7 iirc).
On top of also being why Mythicals are never allowed in VGC/BSS (outside of meme/irrelevant formats like last one of gen 8): they are (almost) all event-locked and often require to phisically be at the event to get them. Many mythicals are actually awful competitively, very few are actually gamebreakingly strong (cough cough Magearna), so it's highly unlikely it's ever been for "balance reasons" as some would want to believe.
It's also why Walking Wake and Iron Leaves were the only non-mythical non-restricted Pokemon not allowed at Worlds: they're indeed event-only and if you missed the 2 raid weekends when they were available, you cannot get them anymore. Though will likely be normally available somewhere in the DLCs and should be allowed by then.
Iirc they did a similar thing in gen 8 with Gigamax pokemon, where they were not allowed (despite actually being catchable in game with a 1% spawn rate or so) until events were made that made super easy to get one, and Gigamax soup was introduced on top of it.
See I think there's a midpoint to be found here
So: I love event moves/exclusive moves of this nature. It's one of the things I enjoy most about collecting Pokemon and I think it's one of the more fun aspects of the earlier games, adding an element of strategy when you consider that some Pokemon sacrifice specific egg moves or abilities for the chance to add a potentially game-changing move to their arsenal. Bagon can't have Wish alongside Dragon Dance, Mewtwo can't have Hurricane and Selfdestruct on the same set, etc.
And yes, it's obviously frustrating to players who didn't manage to get those earlier moves. But in isolation it's not the biggest issue, when it's just one move here and there.
HOWEVER, and here's the nuance: I think that these sort of moves have to have a time limit, so to speak. Prior to marks being a thing, in Gen IV it was entirely plausible and realistic for someone to have XD Pokemon (to the point that people often showed them off in collections, they popped up a lot as trade fodder, and articles on this website and places like Serebii's PotW routinely mentioned XD moves as options). However, by the time Gen V rolled around it became much less likely you would. And those games threw in hidden abilities as another factor, which put a further variable into the strategy pot (want to run Heal Bell on Dragonite? Well then no Multiscale for you).
And so, of course someone might want to use their obscure Sing Marowak or Follow Me Magmar or Selfdestruct Mewtwo as a fun option in a tournament. But GF rightly see this as ever so slightly unfair so they ban it.
So why not balance the scales a bit? If you're going to ban Follow Me Magmar because it wasn't obtained in Gen VI, then
make the move available to Magmar in Gen VI natively. Then there's no issue! My Softboiled Clefable is still a collector's item but some ten year-old from Japan still gets to use it in a tournament if they so wish.
And really, three generations is absolutely enough time for GF to say "okay, this has been event-exclusive or just plain hard-to-get long enough, you get it normally now." There are numerous examples of this happening:
- Rayquaza, Keldeo, Victini, and Meloetta all get their respective special moves as starting moves in SwSh
- Draco Meteor is a TM in ScVi, with no happiness requirement in SwSh
- Volt Tackle can be tutored now instead of only being accessible via breeding
- Formerly needing the Zygarde Cube to learn them, Zygarde now gets Core Enforcer, Thousand Arrows, and Thousand Waves as starting moves, while Dragon Dance has become a TM (still no Extremespeed, though)
- Speaking of Extremespeed, Dragonite now also gets Extremespeed as a starting move (this one's complicated - it only gets Extremespeed as an egg move from itself, but the only Dratini that knows Extremespeed is a gift in HGSS, meaning that there was a period of time in Gen VI and VII where no-one could use Extremespeed Dragonite until Pokebank compatibility dropped, since Kalos- and Alola-caught Dratini had no way of learning the move)
