ST-Key on a US or JPN does what it means to do....play US or JPN on whichever console you choose.
Yet like you are asking, the ST-Key is great for a JPN system if you wish to play US games easily (Since none of the US titles need any of the RAM carts)
Though, if you intend playing JPN games on a US system, please be sure to have an Action Replay 5 in 1 (4 in 1 usually misses one of the pieces needed to play all imports properly)