One question I have is - do you keep track of arrow breakage? Encumbrance? If you house-ruled that stuff away, then that is a side-effect and part of the problem. I've played in both kinds of campaigns, and when archers can be absolutely assured they have infinite ammunition, that gives them a significant power boost.
When they have to be a little careful with ammo, then they will not become one-trick wonders. I've seen that from both sides of the table. You don't want to starve them just to be punitive, but if Wizards run out of Magic Missiles occasionally, Archers can run out of arrows too.
It also means when they're getting low on ammo, it's easy to have them encounter foes with bows and arrows that can be looted (that maybe use the same tactics hehe) - or even finding a dead body with a partial quiver.
Great point, and one that cannot be stated enough when it comes to game balance. Enforcing encumbrance and ammunition is an important aspect to ensure the Ranged builds aren't being advantaged disproportionately.
I'll also throw +1 toward the folks that mentioned using Mounted Enemies in outdoor combats... Cavalry is an essential tool for medieval combats.