Police cars see rougher conditions than standard cars, granted, but this is a non issue.
I've had 4 Crown Vics wrecked out from under me over the years, (drunks, all of them) and department wide we'll have about 5-8 major crashes a year. Also usually drunks. The majority of these happen while working accident scenes, and involve rear end collisions.
(always wonder why the patrol car parked waaay up the road from the wreck is the oldest most ragged one out there?)
We have had one officer die in an exploding Crown Vic situation, Rick Hunter was deliberately run over by a fleeing felon who then hit the patrol car. Speed was 90+, and the Crown Vic was in pieces, some as far as 400 feet from the impact location.
Yes, it burned, but that was hardly the cars fault.
The fact is, there is only one vehicle out there really suitable for police work, and the Crown Vic is it. I don't see much purpose in spending a couple of thousand per car to reinforce a gas tank when the crash has to be so severe in the first place that it's probably not surviveable anyway.