User blog comment:StrategicMagic/Xerath and AD Mids/@comment-9757323-20130519170030

The popular mid assassins are mobile assassins with huge burst, mostly Kha'zix and Zed. Xerath's passive does not make him naturally tanky against physical damage dealers, it just makes him a tinly little bit less squishy against them late-game... With 5 items you'll have a ~50ish extra armor which isn't bad but it's not gonna save you if they come close anyway...

Also the thing about his stun is that's it needs both setting up and landing a skillshot to achieve. If you go aggressively for the stun against an ad assassin with a gap closer(That means you use your W to cast E-Q from a safe-ish range) you leave yourself vulnerable for the next ~10-15 seconds that w/e are on cd for them to use their gap-closer on you and do much more dmg that you did to them. If you save your E for when they come close then you'll still have to land a quite narrow skillshot with a delay on them which is really hard to do on someone who's meleeing you since they can just keep sidestepping when attacking you to dodge it easily.

Xerath fits a poke composition best, and poke is what he excells at. In the poke/tanky/assassin game poke beats tanky beats assassin beats poke. Xerath wants to have a tanky opponent, get a liandry's(and later on a rylai's) and poke him down to 40% before executing him with his ulti, not play against an assassin who'll jump to him and probably kill him because he lacks tankiness and he lacks escape.