I am a supporter of using Grabner more, not sitting him.
There is no question that he has not displayed his normal extreme speed burst in his 31 games this that we are used to. Is he slow now? The answer to that is I only wish that Lee, Bailey, Tavares, Strome, Nelson, were as "slow" as Grabner is right now. Folks, Grabner is still a very good skater, still well above average speed, just not the extreme speed we have sen from him in the past. He has blown past a few defensemen who try to close the gap too much on him.
As for other attributes we are missing out on with him anchored to the bench, Grabner has the most active stick on the Islanders team (one of the best I have seen in the NHL). Even at his less than optimum speed, he is a big threat to teams trying to break out or rush through the neutral zone with that combination of speed and active stick.
I expect that he will recover his speed burst, it is a matter of healing and conditioning. he is only 27 years old and has not lost his speed, just temporarily, but is still a useful play when not mis-managed by a coach that seems never to be happy with him.
Wasted talent of a useful player sitting for a slug like Kennedy. Kennedy is fine for what he was intended when Garth got him...injury insurance.