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.