I have also used blankets to cover gear left in the car overnight in cold weather. Whilst I agree that no heat is generated under the blanket(s) I have often found that, after a frosty night, the blankets will have absorbed the dampness which would otherwise have settled on the keyboards etc.
The equipment underneath has remained dry so I think that there is some benefit even if it aint really a sensible thing to do.

The trouble is that when you get home at 2 in the morning after a long drive and a heavy evening, lugging gear indoors is not allways the first thing on your mind.

Trevor