some may remember the fatal L39 accident at duxford last summer. pilot ejected and was fatally injured, instructor stayed put and walked away. one problem would seem that the average ppl isn't trained (although is prob briefed) on how and when to use them. another is that live ones in crashed aircraft pose significant risk to emergency personnel and accident investigators.
on the other hand, if things did go pete tong while airborne in a hunter/jp/aerovodochody etc, i'd rather just pull the handle and let the accident occur a considerable distance away.
regards, BGPM.