I think they are a worthwhile addition to the overall plan. Yes, they can be defeated, and yes, they can be a deterrent. I dont know if anybody really knows the percentages of either aspect, but seeing many news bits and police comments about stolen vehicles Im going to guess most are not exactly pros or on the higher end of the intelligence spectrum. I do believe most vehicle thefts are from opportunity, not targeted thefts, unless one has nice late model stuff high on the theft list.
Ive had similar discussions about home security. One relative i asked if he had installed longer screws in his door hardware replied "Why, they'll just break a window and get in." Uh, maybe, maybe not. The info Ive seen is most home entries are kick-ins and if that doesnt go well they move on, they prefer not to either make much noise or spend much time getting in, both can attract attention. True pros are a different story but I dont believe most are done by pros.
My take on either is if you can do it, its relatively simple and provides some help and deterrence, why not? A good investment in time and energy.
