Honestly in this day and age of keeping your stuff plugged-in every night the self-discharge of nimmies seems less of a concern. It's only a real pain when you put rechargables into a remote or a flashlight and then find out in a week it's already dead just from sitting and you have to pull the batteries out (which involves screws if it's a children's toy) to recharge them.

But the lower energy to weight ratio would be a showstopper for a car, yeah.