I don't know why you'd use your Ryoma to kill an enemy Ryoma in arena when it's so easy to bait them into dying to your blue unit. That's a sub-optimal attack, regardless of IV.
I regularly place in arena 20 but get smacked down because I lack the merged units to stay. Being a "top arena" player just boils down to having higher merged units. If the game ever puts everyone on an even playing field then we can talk about being a top player
But even assuming what you are saying is true, that really proves my point. That's at the margins, and in a place that doesn't matter to 99.9% of players. That's exactly my point. Do you think having a -speed Ryoma matters in lower tiers? In PvE, which is the vast majority of the game? Probably not. Is it worth it to keep rerolling for a better Ryoma, just in case you one day find yourself in a battle of merged + 7 Ryoma? Probably not
The bottom line is that IVs don't matter in 99.9% of circumstances for 99.9% of the people who are playing the game. Can it tip a match-up against you or in your favor? Of course. But it's not a big deal, and people shouldn't bench units they'd otherwise use just because it's a "bad" IV.