The locks on most suitcases won't stop a determined thief for very long, on the other hand, a strong strap using a conventional type buckle, rather than a plastic 'clip-in' device, will require too much of a thief's time to undo.
In South Africa a favourite for thieves is a spoke from a motor cycle wheel, it is used to bang out the hinge-pin, (see Samsonite and Delsey), the thief then feels around inside for cameras etc. and after removing them replaces the hinge-pin. When the case is retrieved it looks normal and the theft goes un noticed for some time. A strong strap will stop this too.