I agree. D20 or other a lot of other systems do not have a solid and easy mass combat system.
I thought Heroes of Battle would fill that niche, but it didn't. Other publishers have come out with a few, but prove to much bookwork for the GM before hand.
With a mass combat system in place, a hybrid wargame would follow easily. The 2d6 String system could easily be adapted into a miniatures wargame, but a lot of gamers would like to stick to the RPG.
I have come up with a mass combat system for d20 that really gives the feel that a character is in the middle of mass combat without the use of a thousand or even a hundred or even fifty miniatures (we haven't published it yet - still playtesting).
Mass combat is definately an aspect we are going to look as soon as the core mechanics are settled. With mass combat rules in place, a GM or Player could run a war campaign and definitely give an epic feel to an RPG. Also, it creates an atmosphere that the characters are a part of the world, rather than a small, isolated group saving the world all by themselves.
While that is good - it does grow old after the fifth time they saved the world. Even the LoTR characters needed a mass combat or two in order to change the world.