If they are cutting the strap then they will have to go to a bit of effort to put a strap back on (or have a helmet fall off!)
Generally I work on the fact that anyone who was after my pretty average helmet would be a bike rider and hence understand that taking it is very bad.

And while I live in a big city with typical crime levels (and probably well above average car and bike thefts!) Ive never been too worried about the helmet if its locked on the hook.
But Im not totally stupd - I dont leave the bike in bad areas!

Ive also left my jacket with the bike - but I lock it with a padlock and run a coily wire thru one of the arms.