But if I do get you, you're saying that +20/15/10/5 hurts the dedicated melee less than the +20/15/15/15 model?
Yep.
Think of it this way. You've got a level 20 Barbarian, he put his five level up points into Str, he even played a +2 Str Race and of course eate a +5 Tome. He has 34 Str before Mighty Rage's +8.
His attack bonus is: +41/+36/+31/+26 = 20 (bab) + 16 (str) + 5 (enhancement)
The die tends to be accounted for as a 10, which is enough to hit every CR 20 monster in the SRD (min 36 vs max ac of 35). Of course this really is just 95%/95%/80%/55% for hit rate not "auto hit".
Now the less melee dedicated Rogue pimped out Int & Dex, and only has 16 in Str after level ups and only got a +4 inherent bonus off the cheaper graft arms. He has 26 Str and only 15 BAB making his bonus to attack +28/+23/+18 = 15 (bab) + 8 (str) + 5 (enhancement) or 65%/40%/15%.
Under the current system of BAB, the Barbarian is around three times more likely to deal damage in a given round. More often than not he hits fir three of his attacks with little effort while the Rogue struggles to hit with even one of them.
Boo bad system right? House fix, penalties stop at -5.
Barbarian has 95%/95%/95%/95%
Rogue has 65%/40%/40%/40%
Hey, looks like a good fix, now we've got the Barbarian always hitting and this little empowerment also gave the Rogue something right? Except, now much of the Barbarian's hit chance as exceeded the required number to hit, and thanks to this global empowerment the Rogue only needs to hit a +35 bonus to have better hit chances than the prebuffed Barbarian. Which is where the flaw is shown, another +6 to attack and the Rogue nearly "caps" his attack chance. Flanking and Flat-Footed alone almost pull this off. Roll a 5 or below and he could miss still but meh, you're just as likely to roll a 15 for Critical Attack to make up for it.
You're not empowering dedicated melee classes. You're empowering the lesser melee classes Like Rogue, Monk or Factotum. given they
already have incentives other than numbers for attack chance, they flat out become superior. Rogue's SA, Monk's Unarmed, even the Factotum's Iaijutsu/SA/Copy-Wha? provide better damage bonuses than the Fighter's feats or Barbarian's Rage. They get other tricks like skills or utility or the ability to run away while naked. What they were lacking was hit chance, and you gave it to them on a silver platter without so much as asking them to buy and UMD a Wand of Waithstrike. Hell, even Clerics & Druids end up benefiting more from this than dedicated meleers. Grats, your intended buff widened the gap between McFighter and Codzilla even further rather than bringing them together.