I don't see why shadows couldn't have a slider from no detail to high detail and could auto-adjust like the physics do.
It would still be pretty bad, from what I've gathered. And the shadow option at it's lowest quality looks awful.
Even so, shadows don't appear on bricks, so if you have a city with streets then you won't even see the building shadows, anyway. It would be a nice touch, but not as effective unless it showed on other bricks.