I've noticed this response a few times in this thread. Have you guys ever filed a claim and dealt with insurance? Not fun. And good luck getting even book value for your stuff.
I was hit by another drive a year ago and it totaled my 2003 Chev. Avalanche. $45,000 truck and book was only mid $13,000 range. They offered me $11,000. I had to go out and do all the legwork to do comps just to get them up to book value. Then, with the settlement, you get a used vehicle (or sled or trailer) that who knows how it was taken care of it's entire life. Not that I had sentimental attachment to my truck, but I'd done all the maintenance myself (synthetic fluids, etc.) and know how it was treated so it was worth a whole lot more to me than book value.
Also, keep in mind insurance will usually not pay for accessories and add-ons so you may only get a small fraction or nothing if your sled is totaled because it spills out onto the road rather than stays in place. If you make too many claims, you'll get dropped or rates go way up so small claims aren't worth while. Plus, you'll be out your deductible anyway, so another $500 - $1000 that is out of your pocket.
In my eyes, any damage you can prevent and not have to claim is worth the extra 2 minutes when loading/unloading. Heck, one big bump, slide or emergency brake/maneuver might push one of your sleds against the side of the trailer or into another sled. Just cosmetic damage is worth strapping them down, much less denting or piercing the shell of your trailer. Strapping down may not prevent all damage, but when that first time happens (after saying "I've not had an issue and I've been NOT strapping down for years"), you just might wish you'd strapped it down. Of course, that will be like installing an alarm in your house after it's been broken into or had a fire (which, unfortunately, is when most people install such things....AFTER it has happened).
If nothing else, I feel responsibility to the people I share the road with. I'm responsible for my load and doing what I can to reduce damage or risk to other drivers and I expect them to do the same for me. I've had loose items come off of vehicles and hit me in the past such as a spare tire/wheel come down the road at me off a tow truck. My damage due to their negligence. A big pain for everyone involved. That is why I end up checking the trailer hook ups and straps probably 3 times before I pull out of the driveway or parking lot each time.