Nothing is more useful than movespeed. When you're chasing an enemy down in lane, either harassing or trying to get that kill, getting the extra 2-3 attacks in while they're running only makes them wish they could run away from you even faster. When you win a trade, you seal the deal by following up with more harass.
Inversely, if an enemy is winning a trade, you can escape them quickly. If they're melee and you're faster than them, they can't catch you any more. This is why Darius is such a stomper. He moves faster than his enemies due to hemorrhage and it's just outright demoralizing to be chased back to your tower with someone hacking at your heels.
I just love MS quints. Too good on the majority of champions. You don't need that last ~7 AD or so from quints to last hit or trade. What's seven AD when you get an extra two or three entire autoattacks off?
Anyways, GIVE THEM A TRY. You might just never go back to the other quints you've been using.