I don't get that. Free promotions are awesome, and stick with the unit through upgrades into modern times. Samurai has 2 of them.
Maybe it's a "boring" benefit, but the gameplay is definitely there.
For samurai I've always thought it'd be nice if they have a "seppuku" ability. Whenever the samurai is killed in combat, or disbanded manually, it gives culture according to its strength; let the promotion scale with them into infantry to +70 culture/unit (since after Japan was defeated in WWII, the emperor was forced to denounce his divinity, and his soldiers committed seppuku, paving the way for modern democratic Japan, hence, a cultural advancement)
(opposite of jaguars). XD
21 culture per unit might be worth considering if your income is in the red and you just need that extra policy one turn earlier.