Some thoughts on why the rules require posting decks:
The advantage is that it makes the event much more fun to follow. It's enjoyable reading through the results to see how the decks matched up against each other, to see interesting decks being played, and to see if any of the matches should have gone the other way (how disappointing when a player says they lost to RNG if you can't even see the deck matchup...)
The small disadvantage is that it allows players to copy decks, but this is hardly a concern. In Devil's Gate, no two matchups are the same, because every player has a different Allegiance/Support Elements combination. Decks that are highly effective against one player are typically not such a good choices against another. If a player wants to copy decks, it's just as practical for them to grab decks from War or other events. To not have players post decks does almost nothing to stop the problem of copying decks.
The point that posting decks prevents players from using only a couple different decks for the entirety of the event is, in my opinion, actually a point in favor of making players post decks. There's a high degree of skill in constructing a diverse pool of decks under constraints. I'd much rather an event that rewards players who use a variety of decks, than one that makes it easy for players to win with just a couple.
Lastly, there are two other minor points against changing the rules at this point. For one, it's slightly unfair to players who picked an Allegiance because of its deck variety. There are definitely Allegiances in this event which have a much broader array of decks to choose from, while other Allegiances are more confined to a couple very powerful decks. To switch the rules mid-event in a way that favors one strategy is unfair to players who picked the other strategy, especially if they accounted for the particular rule in question. The other point is that it's pretty much impossible for me to prevent players from leaking the types of decks their opponents used, so there's less reason creating a rule that cannot fully be enforced.
I should also comment that the current rules are intended to allow players to wait until the end of the round to post decks, as opposed to posting directly after the match.