cosmetic but does add a little illumination to help you be seen. some leds are harder to see from certain angles. in the daytime.
I agree completely. LED's are great at night and look cool but some I've seen suck for daytime visibility. I can't specifically speak to the ones you reference but some are too close to natural light and just don't show up during the day..
Of course I've taken a few rocks to headlights and got tired of the price so I run a $25 Sylvania Silverstar and love it so my opinion isn't worth much on LED's.
Run what you like. I had one an I thought it lit up the side of the road a little but not far enough out to have any reaction time if something came into the road.
I like'em on my BMW (car) but not really into the look on bike, JMO. They might make you more visible in daytime. Reason LEDs not as visible in daytime is beam is more focused with less scatter (unless you get some real cheapies) and color temperature of the light is closer to daylight so they don't stand out like the yellower halogens, .