I've never heard of them being used on the eastern SEKI trailheads, although I would not be shocked to hear of marmot attack there. The western SEKI trailheads are the only places where it is a known problem. Mineral King, Wolverton, and Road's End are the three areas where marmot guards are possibly necessary, and Mineral King is the only place that I've heard of them being used routinely. In fact, I believe there is a small seasonal store that will rent you chicken wire so that you can protect the underbelly of your vehicle. The rental fee turns out to be about the same as what it would have cost to purchase new chicken wire back home.
I've parked overnight at Whitney Portal about 35 times now, and my vehicle has never been attacked there.
Marmots sometimes get out into open country. However, they tend to live around large talus fields. If you park far away from the talus fields, that will probably do a lot.