I'm going to elaborate on Glacialwrath's post.
I don't main Skarner, but I do main some other supportish junglers (of which Skarner is one, by the way). Skarner, like them, can control a game if he gets a few early kills, enabling him to secure many more kills during the early/mid game for the team's damage-dealers.
That said, once he has his early lead, he needs to allocate the kills to the damage-dealers -- namely the AD and AP carries -- so they can shut down the enemy in team fights. Skarner is, as Glacial said, primarily a utility tank. he applies moderate damage by spamming Qs, but their main purpose is not the damage, as Glacial implied. Their purpose is to apply an aoe permaslow to the opposing team -- which is SO ANNOYING. In that respect, Skarner is a sort of peeler champ, preventing the opponents from reaching his carries and setting them up to perform their combos. He is also an initiator with his ulti or a flash-ulti combo, setting up quick advantages at the beginnings of team fights.
A Skarner with tons of kills is like a Shen with tons of kills. Or an Amumu...or Leona...or Darius...or Udyr...or Wukong. Yea, they're scary, and yea, they can mess you up in fights. Some are scarier than others, depending on their typical roles, but they are all easy to handle if the right precautions are taken, and they do not deal enough damage to carry a team without making themselves extremely vulnerable.
TLDR: Skarner should take a couple early kills because he becomes very capable of securing many, many more by controlling the later early game. But beyond that, a fed Skarner will never, never, ever carry a team to victory because he doesn't deal enough damage. Use him to control the laning phase and set up your carries for a strong mid game, and then build tanky so you can piss off the entire enemy team in team fights while your fed Vayne disintegrates them.