Does the game calculate like that? Or are buffs and debuffs calculated from the unbuffed stat each (wich I believe how it works)?
And even if you are right and the game calculates the way you think, it's a very strange idea that buffs are useless. Trips allready mentioned why buffs are good. You don't need accuracy to land them.
Another decisive argument: They do different things and can be combined. If I want to enhance my damage, I debuff the enemy with def down. Everybody does that and knows that he has to do that allready. If I want to enhance my damage further, I use an atk buff. Atk debuff doesn't help, it reduces the damage I get, it has no effect to the damage I deal. Def debuff + atk buff do more damage than def debuff + no atk buff.
Mathematically proofed that your "buffs are junk"-theorie is wrong, q.e.d.