Why this as a solution, instead of a static bonus to damage for monsters? As I see it, static bonus would have the following differences:
Because I roll dice in front of the players, rolling more dice seems more intimidating and threatening. It's not just about adding the damage, but adding dread and suspense to the scene.
Bottom line: Rolling some extra dice will be more fun for me as the DM than adding in set numbers. Either solution works.
1) It doesn't require rolling additional dice.
Yup.
But, I can already roll more damage dice, just by picking a different monster out of the MM.
2) Unless you're changing minions to do damage dice, you're going to want to increase minion damage via static bonuses in any case.
My minions already roll damage so that players do not immediately know that they are minions.
3) It's more gradual, so you can phase it in over more levels and it doesn't affect things as much all at once. For example, make changes at levels 11/16/21/26 instead of all at once at 11/21.
To me, the big gains for PCs are at levels 11 and 21, so I don't need a gradual solution as much as I need to offset PC gains at those levels.
4) Increasing damage dice has greater effects for some monsters than others. Is it the case that the monsters that deal higher-sided dice should get a bigger damage bonus? That's not clear to me. You'd have some strange effects, like a Green Slaad's (level 18 controller) ranged, dazing at-will doing 2d20+4, essentially the same as the Blue Slaad's (level 17 Brute) melee at-will doing 3d10+10.
Although what you say is true, you are mixing apples and oranges here.
The Green Slaad's Claw attack would go to 3D10+3 whereas the Blue Slaad's Claw attack would go to 3D10+10.
As for the Green Slaad's Chaos Bolt, yes, this is what would happen. There are only two monsters in the entire MM (489 creatures?) that roll a D20 for damage (both Paragon level), so shrug. It's not really that big of a deal. I have to add in another number anyway. Once we drop to D12s and lower, it works out ok. Sure, a few Epic monsters use D6s instead of D8s through D12s, but they tend to be lower level Epic monsters (e.g. level 22). Meh.
Push comes to shove, I could change D20 attacks to 2D10 attacks and add an additional D10 for those two specific monsters and it would still work reasonably well.
5) That said, Brutes should get a larger bonuses to damage than Soldiers under this rules change. Do Brutes do higher-sided die, or just more die/bonuses? Since Brutes hit significantly less often than Soldiers, adding a set +X damage benefits Soldier DPR more than Brutes. Figure something like +2 damage at levels 11/16/21/26, +3 damage instead for Brutes.
Brutes typically do larger dice. So, it works ok. There are some exceptions, but I'm not too worried about that.
Your solution works fine too.