It may depend in part on whether one is playing through each level.
I take it you mean that as in argument in favor of my position? That mixing in Fighter levels (or something similar) is more appealing if you start at 40?
When it comes to combat effectiveness, Bard Song seems to really slow down at 16th level, while the Perform Skill requirements start getting really high...It takes a lot of investment in the Perform Skill (skill points, Charisma increases, skill enhancement feats) and maybe even then a little magical item help.
You're forgetting about Lasting Inspiration at level 20-21.
And there's no "maybe" about it -- you will need magical item help (and/or an environment that fixed Bard Song). 43 "base" Perform, 46 from base Charisma, 56 from Epic Skill focus...need another 44 total. Could get up to 50 from magical gear, but also have Charisma spells/items, Perform feat, and Skill Focus.
Extra spell effectiveness is interesting, but if my concept is primarily a warrior-type, I don't intend to rely on spells for damage. The spells are for utility and defense.
Which is what I was describing. Wounding Whispering is 4 extra damage (assuming Bard 26/Fighter 4/RDD 10, potentially more with less Bard levels) each time you get hit and Ice Storm can be used in a pinch for AoE or if going into melee is too dangerous. Secondary benefits, not the primary focus of the character.
Extra Bard levels make defensive spells harder to dispel- but once 10 levels of Red Dragon Disciple were added to the character, that made the character vulnerable to enemy dispels.
You'll have to explain your statement -- even a level 40 Bard is vulnerable to Mord's Disjunction while Bard 26+ is immune to Greater Dispel and below.
The feats from 8 Fighter levels are significant- that is two feats of weapon specialization, plus either some help in feats to meet the Overwhelming Critical pre-requisites
Why would you bother getting Overwhelming Critical? It's a terrible feat. Only reason to pick it up is if you're going for Dev Crit...and there's precious few environment that both go into epic levels and *don't* ban Dev Crit.
And how many pre-epic feats do you really need anyway? Assume no Fighter levels and you get 7, 8 if Human.
Heavy Armor Proficiency
Weapon Focus
Curse Song
Blind Fight
What else do you really need? Thrown in Martial (or Exotic) Proficiency with Improved Critical if you really want and Knockdown? If you're Elf (since you'd get Longsword and Rapier for free anyway) or Human you still have another feat to play with.
or getting combat-relevant epic feats quicker, and even if a 4th attack isn't a big deal, +1 BAB (at least) is significant.
Key epic feats:
Epic Weapon Focus
Epic Prowess
Armor Skin
...and that's pretty much it.
A level 40 Fighter starting with 17 Strength can pick up Great Strength VII and wind up with 30 BAB + 18 STR mod + 4 feats = 52 AB without a magic weapon or other bonuses.
A 30 Bard/10 RDD starting with 16 Strength (has to get Charisma up) can pick up Great Strength IV and wind up with 25 BAB + 20 STR mod + 4 feats = 49 AB without a magic weapon or other bonuses.
And sure, 1 BAB is significant...but gaining 1 AC and 1 AB at Bard 30 is even more significant. Or if you're comparing to 16 Bard/14 Fighter/10 RDD then you're gaining 2 AC and 2 AB from Bard 26 and Bard 30.
The feats you eventually make up somewhat at the back end- but that starts at Bard level 23 (I assume character level 33).
As opposed to severely diluting the front end? If you want 16 BAB pre-epic then that means you'd have to go something like Bard 12/RDD 4 or Bard 16...so either you're losing out on Bard Song benefits or RDD benefits.
Using those levels for Champion of Torm helps with the feats, saving throw bonuses, and Charisma-based abilities like Smite Evil and Divine Wrath.
Divine Wrath is, unfortunately, Terrible (with a capital T). Smite Evil with CoT <=14 isn't impressive either. Fighter would just be better for the Heavy Armor proficiency, extra feat overall, and (Epic) Weapon Specialization.
Going the evil route, 9 levels of Pale Master offers +6 AC, immunities to stun, hold, paralyze, and the 20th level column on the Bard spells per day. (And that's 1 level short of the 10th level of Pale Master, which offers immunity to critical hits and sneak attacks.)
You could certainly do something like Bard 20/RDD 10/PM 10 with Bard 16 pre-epic -- just means losing out on 2 AB/2 AC for the entire group in exchange for more personal defensive bonuses. But then we're moving into a slightly different discussion than "What mix of classes/levels makes a Bard/RDD (with the possibility of a third class) the best overall warrior?" Could even bring DD into that discussion if we're talking tanky types (Bard 16/RDD 10/DD 14 or whatever).