The conclusion I usually read is that samurai are:
1. Too expensive to be deployed for rushes of weak mobs early in a battle
2. Not bulky enough to handle blocking multiple big dudes later in a battle

So that narrows the situations where one goes 'Yes, a samurai would be perfect here!' to.... maps where groups of weak to medium strength mobs are a concern late enough that 20 something UP isn't a problem?
Not an impossible situation, but uncommon enough such that it bumps samurai down to niche status, I presume.