Ap shaco, although imo not as good as AD, can be a nice counter to AoE mages. Stick a clone in there, and it blows face off, and the AoE oriented mages can't prevent that.
Yi has a good ratio both on alpha strike and meditate, allowing him to tank tremendous amounts of damage given the right conditions. Also Alpha strike refreshes when he gets assits, so this means he can dash it out twice rapidly if a target goes down. Even with moderate amounts of AP thats twice 500 x 4 targets.
Last but not least AP Yi is still good at taking down turrets and has decent AD endgame, allowing a hybrid build if you so desire. Not a gamebreaker, but unlike classic mages he's not easily countered by MR.