The point is that for the same price you can get a naginata. Nagis are way more useful than samurais, therefore samurais should get a small cost reduction as they are not even comparable with doppels.
Samurai are better than doppels IMO. They don't require as many cards to get going, have a higher base damage and siege and still slaughter cavalry. All you have to do is give them some auras and switch the golden pavilion to melee damage and you're set to go...
Bigger area of effect makes doppels better, at least is the impression that I get. Samurai looks like some sort of mellee musketeer, but it doesn't seem to work.