Straight line track speed doesn't always equal "playmaker". I wouldn't really call Dorian Graham a playmaker, and he was one of the fastest if not the fastest timed player in the BE last year.
We often confuse explosive with speed, when in fact they are two different things. Mike Williams was explosive, but not really the fastest player on the field. Same goes for Delone Carter recently. We have had plenty of kids with track speed who frankly were not up to par.
That's also not to say that a FS cant be a playmaking position as well. Look at what guys like Ed Reed, Bob Sanders, Eric Berry, Earl Thomas, Nate Allen, etc have done on the collegiate and NFL level. Having a FS that can cover a lot of ground and make plays allows your defense to do a lot of different things. It make your blitz better, allows more looks, allows the cb's to gamble a bit more, etc.