Myth.
Electricity doesn't magically bleed off into the ground.
It is suspected, but unconfirmed through testing, that the OOOOLD rubber-cased car batteries could drain when placed on damp concrete. It is absolutely not a problem with modern plastic-cased batteries, though the myth persists.... why?
Joe puts a battery on his garage floor, and 6 months later it's dead.
Duhh. It'll be dead in 6 months no matter what he puts it on.
Aircraft are required to be electrically grounded when they are parked.
You used to be able to buy grounding straps for auto's to hang from the suspension to prevent a static shock.
Think about it... your kickstand is metal.
When it's down, it's resting on the ground.
The plate is metal. Put the kickstand on it, and you simply have a bit more metal between the frame and the ground... it is no more or less grounded with or without the plate...
But even if the plate were electrically grounded, you need two contact points to complete a circuit (thus, draining power).
Use the plate.