I've never had an issue with one of those jam nuts coming loose. Torque to spec. A couple drops of loctite would be fine for peace of mind, but I don't use any. You'd actually need to have BOTH jam nuts come loose for the setting to change, and then the adjuster would have to turn too.
And yes, the adjustments do help a lot in the real world. The camber bolts are only good for like +/- 1.3° if I remember right. Which is why without offset bushings most folk struggle with getting more than a couple degrees of + caster. With the SPC control arms you have a much larger adjustable range. Like I run +6.5° of caster and -1° of camber and I have a ton of adjustment left still. And changing the alignment is a lot easier.
Do I fool with it now that I have it dialed in? No, not really. But I'm not doing events or track days. It's something you could change from one track to another if you knew the track and your car. But, in the process of dialing it in I've run everywhere from -.5° of camber and +3.5° of caster all the way up to -1.5° of camber and +8° of caster. So I settled on my current specs after running through a large range of adjustment. Now granted not all of that was with the SPC's, I started with 73+ UCA's and offset bushings and used a non adjustable set of tubular UCA's before going to the SPC's. But with the SPC's I could have done all of it without ever changing out suspension components.
Nothing is really "wrong" with the MII spindle, it's actually a super adaptable suspension which is why it's been adopted by the hot rodding world so wholeheartedly. But it's the wrong question - what's wrong with the '73 A-body Mopar spindle? Or the FMJ spindle? While some of the aftermarket support is new (which is awesome!), there are lots of brake options for them now too.
So, what is the '78 MII spindle doing for you that you can't do with a '73+ Mopar spindle? First one is easy, it's set up for rear steer so a rack and pinion works without terrible Ackerman or an impossible steering shaft angle. Got it.
But what does the camber gain look like? How does it change the roll center? What does it do for your bump steer? Or yeah, turning radius?
And if you're using the A-body UCA mounting points and UCA's, well, you're already keeping a significant amount of Mopar suspension geometry. So what is improving, other than getting a rack and some header clearance?
And that's the thing I don't see anyone talk about. Or publish, for that matter. If the suspension geometry was so much better, why not publish that information? Why not have detailed analysis of all the ways a coil-over conversion will handle better?
That's the bottom line for me. A coil-over conversion gets you a rack and pinion and some header space. The downside is it loads the Mopar chassis in a way that's different than what is was designed for, and that's not a small thing (although it can be addressed). Are there some geometry changes? Yes. But like anything, there are pros and cons. And that gets back to what Peter said, most of the people buying HDK's, or AlterK's, aren't doing it because they understand the differences in suspension geometry. They see a rack a pinion, they see coil overs, and they're more familiar with those things than with torsion bars. They see a complete package they can just write a check for instead of doing some research to understand the suspension they've got and what they can do to improve it.
And I'm not trying to single out the MII spindle, the same can be asked of the Corvette spindle based suspension too. What does it actually get you? I can get 15" rotors with GT500 calipers for a 73+ Mopar spindle if brakes are the only thing. How does that system improve the suspension geometry, and therefore the handling of the car? Or, is it just shiny new parts that sound cool? "Yeah, it's got a full suspension conversion using Corvette spindles". Cool, but if it doesn't handle any better than a Mopar with torsion bars and 73+ spindles, what's the point? Although I'm sure there are already B/E body guys lining up to buy that suspension, got to be the most expensive, latest and greatest. And very few of them probably have any idea about why it's actually better, IF it actually is.