I used to wonder the same thing. But today, I wouldn't buy a stainless light myself. Too heavy and for no good reason. Remember, many lights today are hard anodized so are quite tough to scratch or dent, and many have stainless bezels. I wouldn't even buy a titanium light - and for the same reasons. Heavy, more costly, and the only benefit is its shinier.
Also, with the hi-power LEDs today, we need more heat transfer, not less. So now with stainless, we get a heavier light for our EDC pockets, a more expensive light in a poor economy, and it makes the LED run hotter and with a shorter life span. The outside of the light would be shinier but the light output would eventually be dimmer.
Shinier but dimmer and heavier or....black/gray but brighter. I know which one I'm going with.