You're not taking into account reliability.
Yes, if you're doing AD skarner, when you do well, you do REALLY well. You feel like a God. You get those quadras and those pentas and w/e you want, really. What you don't get is consistency. You'll get those games every once in a while, but most teams with any decent players shut you down hard with any decent cc. Tank skarner, however, has the survivability and follow through of... well, a tank. He can fight through as long as he needs to, and lay down stuns all over that fight, as well as being able to rush through to those carries, especially if you carry flash for that sexy sexy ult.
Long story short, like I said, AD Skarner is fun enough, but it's incredibly inconsistent. When you get behind, when you don't do well, you REALLY don't do well.
It works alright at low Silver and you'll sustain yourself fine, but you'll find yourself climbing and continuing to win if you drop that Tank Skarner into your pocket instead.