well i dont think he outlanes many champions. his harass is certainly weak, and he can die very easily with any amount of attention.
but consider this: if u are controlling your opponents item builds with a single champion pick, you are doing something right. forcing opponents to buy banshees veils is putting a lot of gold away from more damaging items.
your ultimate also automatically gives your lane partners advantages in lane. the other team knows they cant take too many risks because your team has to do less work to get kills. and if the other team disregards your ultimate, then a risky play on their part which would normally not hurt them can turn into a lead for your team.
also hes an amazing teamfighter. probably causes the most damage of any ap carry, and if karthus can postion himself in the middle of the enemy team, turn on his e, wall, die, ult, he does MASSIVE DAMAGE to the enemy team. at that point, banshees veils are probably gone for them anyway.
banshee is a good item i guess.. but so easily removed..