In this day and age, I don't think a trailing spouse is any more or less likely to either stay or leave a job position than any other employee. Even as someone who has lived around the same area for 38 years, there is always a chance I will have to move due to DH's position. It is a bigger factor now that you have dual income families.
Also, any employee you have could get hit by a bus today....or get in an accident that places them out of work for a significant period of time.
Succession planning is important no matter what the position. There are no guarantees. Cross training employees can help a bit also.
So no, I wouldn't ask the question.