User blog comment:Myself v1.0/AP on-hitters/@comment-4996845-20140213150338

The main problem with APAAers is this: more often than not, they need to get into the fray and stay there for a reasonable amount of time to deal their damage. While there are some that can get away with that with clever positioning and/or escape abilities for times when things get hairy (like Kayle or *mr-global-taunt* Teemo), they can never attain the staying-power level ADCs have through lifesteal. On the other hand, while they will possess above-average burst, it usually won't be the magnitude of a squishy instagib (Riot makes sure of that - note the recent nerf to Kayle's E) - so they cannot rely on the "drop my combo and GTFO" strategy for squishy AP carries.

Consider this: the combo that you mentioned, Morde's NT+LB AA into Q. With LB's cooldown, you'd have to stay in melee range for at least 2 full seconds. That's more than enough time for an ADC to cut up his metal hide into razor blades, unless he builds some resistances to throw on top of that. And this is where slot efficiency kicks in: to burst, you just need AP. To deliver damage over time, you need resistances, and neither LB nor NT give you any.