I once played an almost all-hero battle for fun. It was 700 points of Rohan heroes vs about the same in dwarf heroes. Everyone was on foot, and we fought a battle to the death just to see what would happen. I know it's an extremely bad army list, but we were just doing it for fun (kind of like the Sauron vs all the Ringwraiths battle....the Ringwraiths destroyed Sauron.
If only they had teamed up against him in the books....)
Rohan had Theoden, Eomer (Knight of the Pelennor), Theodred, Eowyn, Erkenbrand, Gamling with Royal Standard, Hama, Grimbold, 2x Rohan Captains, 2x Royal Guard (to fill out extra points).
The Dwarves had every single Dwarven named hero plus a couple of Khazad/Iron Guard to fill out the extra points.
So it was something like 9 Rohan heroes (and 2 RG) vs 7 dwarven heroes (and a couple soldiers).
The Dwarven heroes are much more powerful, but I was hoping the numbers and Royal Standard would help.
In the end, Rohan got slaughtered. The only Rohan hero with F6 is the buffed up Eomer, and only about half can get D7. The dwarve's lowest D was 6, on the Iron Guard, and highest was 9 on Dain and Durin. Most of the Dwarves had F6 and D8+. In addition, most of the dwarven heroes had combat bonuses (+1 to wound, reroll a die, etc.) while Rohan has nothing of the sort. The standard allowed me to recklessly use might every single turn by multiple heroes, but ultimately the Rohan heroes were just too outclassed. Eomer went crazy pretty early on, and actually did a lot more damage that way (S5 helped, though I had to charge whoever was closest). His 3 wounds and 3 fate actually kept him alive almost to the end, despite D4.
Anyways, yes the Royal Banner is nice, but planning on exploiting it as a main strategy doesn't work very well. In a large battle, the standard with a couple heroes around it would be useful, but definitely support them with troops. And one might per turn is only so useful when that's also the max you can use per turn and the heroes receiving it are generally medium-low level heroes. For most circumstances, the banner with heroes are just too expensive to invest in.