Interestingly enough, aside from Fiendish Codex (which is an awfully weak item) there's no +AP/Cooldown Reduction item that isn't laden with other stats, either. Frozen Heart (and its components) are expensive armor bits, the Tooth has attack speed, and so on. So having damage, armor penetration, and cooldown reduction on one item is a pretty good deal for melee since a caster wanting CD reduction almost always has to settle with paying for a ton of other stats too on the side, which may not always be useful.