Just put a "baby onboard" sign on your MR2. That will definitely make people act more prudently behind you.
But, seriously, you guys are making this more complicated than it needs be. It is a fact that a reduction of velocity between two converging objects equals a reduction of collision force. It is also a fact that car manufacturers separate the metal frame from the occupants with as little as less than an inch of material including plastic and foam. I don't need Aristotle to write a treatise for me to convince me that covering a metal bar one adds to the cabin with some foam is a good idea that will do more than nothing at all.
Whether a roll bar is needed or not is a subject warranting examination of circumstances on a case by case basis revolving around how the vehicle will be used, weather, and geography. I'd guess, as a general rule, one's odds of being involved in a rear end collision are much greater than his odds of needing a roll bar. As another general rule, if the car will ever see a race way then a bar is mandatory, whether or not rules and regulations require it. There are a lot of interesting ways to die under the crushing weight of a car that do not involve the relatively quick and painless shattering of ribs, femur, skull, and ruptured internal organs that accompany an event similar to jack stands failing while changing the oil. Take, for example, the possibility of rolling over onto a muddy embankment, with the contour of the soft, squishy surface pressing slowly into the cabin space as the car settles deeper and deeper into the mush, inevitably (and slow as a baby's crawl) closing in on the occupants, with time enough for regrets of many decisions, not least of which the decision to forego a roll bar and padding because lolmyhead.
