When Spintires came out, you could drive fast. It was risky, you risked toppling a loaded truck, you risked getting stuck badly, maybe hit a rock and be stuck with the wheels off the ground, take a wrong turn and fail, but being skilled you could hit AWD and diff lock and 1+ gear and go zooming across the map, rescuing stuck players in multiplayers quicker than they thought possible, in hard mode of course.
The diff lock damage was introduced, and the same playing style was heavily penalized: go fast on most surfaces, and the diff lock damage would happen, and at times when you're most concentrating about your driving. Having to watch that, turn it off for 5 seconds, lose speed at a critical time, could upset your strategy. (Yes, lose speed: that means wheels are actually spinning when you turn it off, so it shouldn't get damaged in the first place, not being able to build that kind of tension.)
Diff lock damage is the game designer deciding, "you don't get to drive fast in this game", and that's that. The game I bought and the play style I loved is no more. Spintires is now the game of slow driving, to the point where I defy you to get the UAZ stuck unless it is either under water or sitting on a rock with all wheels in the air: if you stick it in 1/3 gear 1, it will always pull through.
But now it takes ages to reach stuck trucks for support. Voila, here comes the new checkpoint system that lets support trucks pass on shortcurt roads. Navigating the terrain at speed is no longer needed. What was the game about again? Should be called MudCrawler, not Mudrunner, that'd be more honest.
Not only would the checkpoint system not be needed if there was no diff lock damage, it makes the maps worse. When you have a road across the map that you can't use for lumber that is also fenced off against use from the sides, that strongly restricts the possible routes that you can use. You get less challenge on the same map area, and it is obvious when comparing maps to their orginal counterparts.
And yes, I could play casual, but have you been to casual multiplayer? have fun getting people to stick around and not recall their trucks, unless they're fully loaded and at least halfway to the lumber mill they'll rescue to garage before you arrive.
Diff lock damage doesn't actually add anything to the game thatwould be worth while, it's an annoyance, nothing more. You can always avoid it; the truck stalls if you don't hit Q quickly enough, so you will always want to avoid it; it doesn't add depth to the game in any way I can see.
The upside: trucks with no differential (but where are the chains on the Zil?) and trucks with always-on differential that take no diff damage.
tl;dr Diff lock damage slows the pace, and necessitates kludges that make maps worse, with no upside.