...Locking Extra Deck slots behind a paywall is kinda 🔥. I hope either they're not usable in PvP (since that would give a HUGE advantage, significantly moreso than EX Structure Decks), or can be bought with gems or coins or something, not just RL cash. Also, regardless, hope it isn't on a per-character basis like the deck slots are. That would be painful.
It gives the player more options. Even if that's just 3 Armades instead of 2 or something, that is still a pretty big advantage. Also, realistically, most decks will either use 0 Extra Deck monsters or want as many slots as possible. There aren't THAT many (beyond maybe Masked HERO) that wouldn't be at a disadvantage if they stuck to 5 if given the choice.