Choice scarf is usually used on pokemon with mediocre speed, since nobody expects them to be fast and yet it surprises faster pokemon by outspeeding them and knock them out. Starmie is already one of the fastest pokemon around so there aren't a whole lot of pokemon it can surprise, and without that you're basically outspeeding people you normally outspeed anyway, but now you're losing the versatility to use different attack moves.
For the reasons stated above, Starmie would be better suited for Choice specs, if you're going to use a choice item. I personally prefer Leftovers.