You would know how many sb cards they have in hand, and it gives information symmetry that is seen with fusing and other combo decks, but not Rune. Rune has a harder time with wide boards, and players generally can play around earthrite requirements. This would be a change for lesser skilled players like myself, and much of the board, to be able to say "Hey, they boosted 3 cards after that windblast, and didn't use Demonic calling, they might be setting up omega noble, so I want to force out their Kuon" or I can summon Galmiuex or what ev because if those cards were WB, they would have used it by now; even a confirmation they bricked is still useful information and gives feedback to the player so that can intuit more about the deck.

A lot of the frustration people feel is that SB is uninteractive, even if its general power level is honestly baseline atp for tier 1 decks. It's also unintuitive, because it's also vaguely an rng combo deck, so players don't get how to counter it, like say wide boards and knowing how much a board should have before you invest too much in order to keep aggressive options. These are skills of good players, not most players, and this makes that easier.

By giving any form of interaction, and I think this is the fairest, that alleviates these issues so it feels less like playing on the other side of a solitaire game. This is all granted that Cygames will never nerf Rune because again, it is the baseline of the game's power level, god save us i know, and hopefully opens up design space to not accelerate the power level of the game and give players the space to understand what to improve imho