IMHO, it depends on what you mean by "tank". When you read that xxx is someone's tank, that person probably means that they use a single figher-type (with the lowest AC possible) to attract all or as much as possible of the enemies' attacks, while the other party members use their ranged weapons or hit and run attacks to dispatch the bad guys. In this scenario, the tank isn't really used to attack but rather to distract.
For more on this, see SixofSpades' site:
http://www.geocities.com/sixospades/bg/chap9.htm