Okay now that I'm out of X2 and back to Splat I finally see what you're talking about. I actually can't remember if the duration box went to gray in Splat when the smart tool is chosen like in X2, but everything works the same, so I guess it really doesn't matter. Someone may have decided it made more sense to keep the duration box highlighted since if you alt+click with the smart tool it will insert a note of whatever duration is in that box. And you can still click+drag a note to whatever length you want. So basically I would say don't worry about it.