The thing is, most policies say that if the bike's not secured with a lock of a certain anti theft rating, the policy is void. So having it nicked while you buy sandwich etc wouldn't be covered unless you lugged a D lock around with you, which no one does. I doubt many people ever leave their trials bikes locked up for long periods outdoors anyway?
The insurance I had for my XC bike required me to buy a Kryptonite D lock and send in the receipt in order to make a claim...