not every samurai was heavy armored, most of them was using chain mail, that wasn't really heavy armored armor like the paladin and such.
plus, what will bring as gameplay the samurai to the tanking? nothing news... it will still be a physical tank with a 2 hand weapon mostly relying on the parry... what we have already with the drk and war.

plus a tank is not forced to have a heavy armor, they can perfectly create a tank purely magic that will use different spell as defense...

believe me, even if they add sam as tank, the DF situation will not change! because people simply don't like to play tank that was always like this and will never change whatever you try!

ps: and anyway, it's useless to talk about this, the director of the game (yoshida) have said that for him sam is a dps... we can argue he is still the one deciding not us.