User blog comment:The5lacker/Varus is not a normal AD carry/@comment-5761259-20130613185728

What you are refering to is hybrid. Hybrids excel at making the best useage of of all their scalings, which means that they have exceptional early game damage. However, if you build a hybrid champ that focusses on AP, you loose out on crit chance, or you waste an item, since crits get better the more AD you have. Likewise, if you focus on AD, you loose out on the burst and the fact that AP is cheaper than AD.

Also, you can't choose to penetrate with both, or any sometimes, of the stats, meaning that you don't get amazing late game damage. What you are suggesting is Hybrid Varus, not ADC Varus. There is a discussion about AP Varus, and why it isn't ideal on his page, and most of those things apply to hybrid. You cannot ever deal the insane burst of a tripple procced w with 600ish AP, or the mad autoattack damage of strait AD Varus.