Pick me out a tank.

quote Lil J

Nasus is viable if you're good at last hitting. His passive + Q can negate most harass.
Then when late game comes, you walk up and pound their faces in.

quote A Wind Powered Moose

quote Lorx

Some teamcomps don't need one, however most still use some kind of mainline tank. Tanks are very, very useful. Especially ones that are also half-decent at dishing some kind of damage. Champs like Udyr, Shen, J^4, all have the ability to tank very well and still hurt. It's very valuable to have cc-bot that can keep people off the carry, and initiate teamfights under multiple scenarios. Generally speaking every pub match needs one since running without that solid cc/initiate, in favor of something more specialized, will never end well while dealing with pubs.

That's my point though, people like Udyr, Shen, and Jarman are all tanky bruisers. There are very few pure tanks in the game. Leona, Alistar, Sejuani, Rammus, and Shen before his buffs are the only ones I can think of right now. Everyone else can do mondo damage while still being unkillable pieces of shit. Almost every bruiser in the game has CC that is conducive to managing teamfights. Itemizing for pure survivability, as a tank would, is just plain nuts-on-chin stupid.

Unrelated and not my opinion, but by purists standards, you can only be a tank if you have a taunt. =/