How I believe GA and pierce should work together

[Isolde]

Unless I'm mistaken, there is no official line from Blizzard saying that GA piercing is a bug

Ah. It's a bug (well it was intentional to make the change but not intentional that it actually get shipped) and will be removed in 1.10. Officially.

Same, nowhere on Arreat's Summit it is said that pierce skill doesn't work with guided arrow.

The purpose of the Arreat Summit is to reflect how the game *is* rather than how the game is meant to be. :lol:

Now, my key argument : in the name of basic logic, why would a level 18 skill (guided arrow) take precedence over a level 30 skill (pierce) ?

No idea :) I don't think it's actually taking precedence either - that argument would only apply if pierce was useless if you had points in guided arrow, but it isn't: it still applies to all your other arrow skills.

PS. Sorcs weren't nerfed via casting delays... they were originally added to prevent spamming and frame rate drops on slower machines. At the same time as casting delays were added, the damages of the skills with casting delays was increased significantly (which contributed to uber firewall, etc). At any rate, if something needs to be nerfed, there are many options with which to do it. Just because one might be used in one case, doesn't mean the same avenue will be taken in all cases.