LED bulb with slow on motion detector
That's a cool idea but Google won't show me any commercial products like that, outside of commercial lighting intended for displays.  Are you quite sure this actually exists?  Make/model maybe?  I'm wondering if it's actually the lamp doing that for some reason. 

It would not help extend LED life but would make them more pleasant. Look at the documentation and see what it says about working with LEDs, that is to say, there are several methods of dimming, which one does it use?  Get compatible "dimmable LEDs" and you should be fine. Even if they are not compatible, they'll come on when it ramps to full brightness.