some things that come to mind with this question of why it is so easy to obtain GW,
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a lot of hyper carries in this game usually build some sort of life steal item early, master yi, fiora, vayne, there's more of corse but those are the ones that immediately come to mind. In order to prevent these champions from getting really big really early, (before they should be big) we need a cheap item that you can get in the early game to keep pace with the insane amount of life steal some of these champions get.
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As far as I know,
is not in any champions recommended build, and so people don't actually know about it unless they've done some research or have played a little while. (you will almost never see this item built in bronze/silver, and it is still rare to see in gold, not sure about plat+)
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The base stats of the item are not that good, its basically just a slightly more powerful long sword. (a little math to follow / illustrate point)
a long sword costs 350 gold and gives+10 attack damage
800 (cost of executioners calling) - 350 = 450
So for 450 gold, you are purchasing, 5 attack damage, and the Grievous wounds ability. The item then only has ONE build path, so you are have no flexibility at all with item build with regard to this item.
As for GW on mages, I don't play mages so i'm not gonna try to justify it there =p