I just strapped my peli case to the luggage rack when I toured with my 4/4 - it never occurred to me to lock the case to the rack, touring in a convertible car is an act of faith to a certain extent any way. The tea leaf frustrated by a case locked to the rack is just as likely to slash your hood either to steal something out the car or as an act of malice
A locking strap will stop the opportunist thieves. And I reckon most thievery is because it was tempting and the opportunity was there, a locking strap could certainly give one piece of mind to a degree.
