I know DDO is not pen and paper, but it seems like it should try to stay true whenever is easily possible. So I for one think Luck bonuses should stack, like they do in pen and paper. As far as I know the only luck bonus items currently in the game are Stone of good luck, Rabbit gloves, and the Head of Good Fortune so this does not seem like a tough change to make.
It may seem like an inconsequential thing, but many DDO fans want the game to be as true to its heritage as possible. No surprise there.