I would agree with the small engine / opportunist arguments - it's not like someone is going to nick your 100+Hp engine on a whim - it's not like you can just chuck it in the back of a car in a couple of minutes - whether you have a chainsaw concealed in your back pocket or not!
Like all "security" measures all you're doing is putting off the opportunist. If they have their sights on your engine / car / plasma TV, they'll take it. I was amusingly reccommensded by my house insurance peeps a few years ago not to bother fitting an alarm, simply because if I were out in the back garden for 5 mins and someone nicked my telly, I wouldn't be covered because the alarm wasn't on! No alarm made about a £2 a year difference to the contents premium!
But yes, I do agree you have to give your insurance co as few excuses as possible for not paying out! My old boat was on 3rd party only - I reckoned 3 years of fully comp premiums and I'd have given them enough to replace it if it got nicked! Admittedly it's value would probably (just) pay for a tank of fuel for some of you.........