Originally Posted by Drip

High speed helper squads make great tanks for lower lever players to race through some maps, but for three-medalling, they're terrible, since they increase the chance of your regular units not even reaching some pests before the speedy helper hits the flagpole. So great for clearing, terrible for completing.

agreed! But not only that, high speed squads are also great for grinding stages. As an example, currently all I do in this game is farm 12-5 so I just place a super strong and fast squad that finishes the stage more quickly and saves me from complete boredom. So friends with helper squads of 700+ speed and 70k+ power are a must have on my allies list
But even if your power is lower than that, it's still useful for farming lower level gardens or weekly stages