Before I type this, I will say that my experience with lockers and tire chains is with 4WD civilian trucks rock crawling or in mud and not MVs, but I think the basics are pretty much the same, but the application can sometimes be very different. With that said, I think they are both useful traction aids but chains are not always practical. Chains can be very dangerous on rocks since you can sometimes get most of the weight of the vehicle on one wheel and therefore hanging on one crosslink. There are no chains made, to my knowledge, that are capable of supporting the entire weight of a vehicle and breakage is inevitable. It becomes a safety issue when you add wheelspin into the deal as sometimes that is all you can do to either keep momentum or floatation. I realize that this probabaly doesn't apply to most MVs due to the weight and low gearing.
Lockers are a bit more versatile, especially selectable lockers. They allow true 4WD as a vehicle with front and rear lockers has all 4 wheels pulling equally. An open differential will send all of the available power to the wheel that breaks traction. This can make it hard to regain traction and can kill any momentum. Applying the brakes lightly is known as the "poor man's locker" and can work, but it is also the quickest way I know of to send the center pin completely out of the differential. I have done that myself, twice. Both times with a C-clip axle, not fun.
In short, they are both useful, but they are very different. Chains are frowned upon because of the noise, the risk of body damage, road damage, risk of flying debris if they break, and the manual labor involved to put them on. They do dramatically increase traction in light mud, snow and ice but they are not easy to put on and take off and since the can be dangerous in some situations, they are pretty much forgotten to many wheelers except for snow and ice duty. Lockers really have few drawbacks. They can change the driving characteristics on pavement but they are easily compensated for. They can cause added tire wear, but that is considered a small thing compared to what you gain. Some say that break parts, but I disagree with that. Generally it's trying to do something your vehicle can't do that breaks parts, the locker simply allowed you to get to that point, which you couldn't have reached without the aid of a locker. Again, they are both useful but the locker is much more useful in more situations where chains can have several drawbacks. Just my .02
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