Limited trunk space is a big deal for some departments. It was the one reason the department I retired from resisted replacing Crown Vic's for a very (and too long) a time. One requirement they have is a full size spare. Doughnuts or using squirting liquid foam do not count.
The gear required in my previous department at one point was so extensive, I asked the quartermaster when were we going to get issued a trailer to bring it with us. With just the required gear, I have very little room left in the back of a crown vic. However, times have changed and the required gear has too.
What this will also do, besides the durability testing, is provide a look into the safety of the vehicles. Unfortunately, police officers get into a lot of high speed wrecks with the cars loaded up with lots of gear/weight. If you research it, civillians driving crown vics had very few cases of them exploding in rear end collisions, whereas, police departments had many cases. It was finally discovered that gear in the trunk was getting pushed through the gas tank and the extra electronics in the vehicle were providing an ignition source. Purpose built police vehicles help to minimize this.
There are a multitude of reasons while on duty the police car needs to be on. I agree that law enforcement is a good case to study hybrids. Ran many a police vehicle to over 100k miles on the odometer, but it really translated to over 400k miles if you include idle time to miles formula.