Most of you guys can probably school me on the details of the Wheatfield, and since it was such a chaotic and confusing section of the battle I probably chose a poor example

Also, to my understanding of how events transpired, Winslow was too far from the stone wall to effectively employ canister. Instead, the battery used a combination of solid and case shot. The solid shot was fired into the woods immediately behind the wall. The solid balls would rip into the trees sending a shower of wood splinters and large branches down into the infantry using those same trees as cover.
I think that most of us fail to recognize the sheer terror that artillery caused. It's one thing to see a demonstration of these awesome guns, but to be on the receiving end of their fire was often enough to put even veteran troops face down in the dirt. Of course they would not be able to make a casualty of every man in a regiment, but their capability to wipe out large swaths of men in one fiery burst was enough to make even large numbers weary of assaulting an artillery battery. I can only imagine how horrific it must have been to see a discharge of canister on a line of infantry. It was probably as much a psychological weapon as a physical one.
Here is another, more general, way to explain it. Artillery pieces were extremely valuable. Both the USA and CSA committed large numbers of men, material and money so that these 2 ton pieces of bronze and iron could be hauled all over the countryside. This wasn't done just to get some random, long-range shots in on the enemy. This was done because artillery was damned effective.
I suppose my whole point is, please don't reduce the capabilities of the artillery just because some people misunderstand its effectiveness.