I'm also not a fan of the magic bonuses, because they're, well, magic. But the nature of cRPG raises certain issues...
I like the shield-wall mechanic, because it makes sense. As you get on-line with your comrades in arms, you might think Our shields are overlapping, plus I feel better that my buddies are right next to me.
However, whereas real life overlaps pleasantly with the shield wall magical bonus in crpg, it does not do so with archery. The advantage given by clumping archers IRL was one of coordination, whereby a group leader could verbally direct his subordinate's fire more easily. But communication in our game is instant and works regardless of in-game distance between players. Coordination between far-flung archers is possible, so that archers can scatter all over the map and still call their shots, concentrate their fire on specific targets/groups, or plan their movement.
Right now, regarding damage output, there's rarely a reason to lump archers together. Occasionally lumping exploits some sort of angular advantage over the enemy, but rarely is real lumping necessary. What you, chadz, are trying to do is force archers to do something that goes against the grain of current cRPG mechanics. Slapping a magical bonus on the problem ignores its source, which lies in the nature of the game we play.
But what's the alternative? I think exploiting the other aspects of this game, and something subtler, like (as has been already suggested) limiting the amount of arrows carried, and making archers less Ethiopian in their running ability. Also, the dynamic of player class interaction (I'm gay) can help here. Scattering your archers can influence their survivability:
1. Scattered archers can get rushed by cavalry, which right now depend on speed and surprise, rather than HP and armor, to kill their victims. The two means to cavalry ass-kick--survivability and surprise--are at odds right now. The faster horses are cheaper than the survivable horses.
2. Scattered archers can get rushed by shielders. They will kite and scatter even more in response to an infantry rush, which distracts them. Enter surprise-attack cavalry. Or, just keep them running.
3. Any archers are vulnerable to other archers. While harder to hit than if they are clumped together, scattered archers are more likely get killed one at a time by dedicated anti-archer archers.
tl;dr: Rather than wizardry, do something less homosexual to nerf ranged. Work with the game we have, rather than against it. The alternative is fundamentally altering cRPG at its source.