For an EV, they could be useful in stopping/starting. Rather than having regen go to the battery you could first send it to a capacitor which then trickle charges the battery (reducing wear). And rather than pulling directly from the battery, you could pull from a capacitor first to get to cruising speed. That would allow the battery to slowly discharge into the capacitor rather than going directly into the motor at full force.

You'd play to capacitors high cycle life and fast charging/discharging capabilities rather than their density.