Everybody wants to be a ninja, most people have no idea what that means...
I leave it to the people who do, and happily play my spell casters instead.
Tanks have their role - there's got to be somebody who gets to spend half the game dead, and you don't want it to be you. So you get a tank. In DnD this was the guy who was rolling up a new PC every game session, so that I never had to.
Rogues, Assassins, etc... are handy to deliver that critical blow before your tank goes down. A lot of DnD people want the tank there, but most of them start to realize that the rogue is more valuable in the long run.
And tanks have a very quickly diminishing return. Their only purpose is to soak up damage so you don't have to - and you only need one guy to do that if you can position him right and keep the aggression on him. A second tank just means one less person with actual versatility. But a team of 'strike ninja's' works great if you know how to use it and you have that rube, er tank, in there to take the attention away from you.