For myself it is a no brainer.

If you feel that there is a chance that sliding a tub over a probe may pull it free. If you feel there is a low chance of a T-stat and a mechanical relay both failing at the same time. It is likely that a Herpstat 1 or higher does not need a fail safe. The VE series for example would need a failsafe so that makes a VE 200 (and a VE100 as a failsafe) very costly 200$ compared to HS1 at 139. I still use a failsafe on my newer herpstats because I can, but I am unsure if I actually need them.