In pro play, Varus has been played quite a bit recently. Xerath has 3 abilities that are long range, while Varus only has 1. AP has void staff, while AD has no % arpen. Xerath would certainly do more dmg than Varus, especially late. Why isn't he picked? Varus CC ult too stronk?
EDIT: % TOTAL ArPen, %Bonus ArPen is bad vs squishies, which you are generally after
TehNACHO2 voteswhile AD has no % arpen
Last Whisper and Black Cleaver?
Anyway, Varus being an AD Champ opens different champion selections for the rest of the team. If both the Jungler and the Top Laner are primarily magic damage, Varus Mid rounds them off better than an AP Mage. His Q also has more range than Xerath's Q. His Ult's also pretty valuable as you pointed out.
I think Xerath still has more raw firepower than Varus, but if you needed the utility of a Ranged champion mid, Varus does better imo.
The Bearded Bard1 votesVarus has an aoe lock down ult, GW on his E, and max health damage on his W.
YuGiHo1 votesum, 
exist
