There's an efficiency benefit, because with an LPR, you're using lower-pressure air to move your bolt. In Mike's reviews, the 2011 Reflex Rail shot 7+ pods from a 68/45. The DM13 shot 10. 30+% better seems significant to me.
The pressure of the air being used isn't particularly important - what's important is the force being exerted on the bolt. If you do the math out (F=P*A and V=A*L) you'll see that to get the same force from two different pressures requires the same amount of air from your tank (neglecting a few details which actually tip the scales in favor of no LPR).
The differing chamber volumes and operating pressure between the DM and PMR alone make efficiency numbers uncomparable.
The LPR keeps you from accidentally over-pressurizing your solenoid. That's one reason the vanilla, (non-Reflex) Rail uses the slower, more robust pancake solenoid...so new players (the Rail's target market) don't blow their noids.
The original reasoning for the LPR was that the solenoids people wanted to use simply couldn't run at the marker's operating pressures. When guns were running 200 psi it was hard to find a solenoid that would run at that pressure. It is, as you say, also a safety feature - you can't adjust your HPR and blow your solenoid. But you can still adjust your LPR and blow it. The other reason was the regulator scheme in paintball hadn't quite been worked out, and regs were less than reliable. Putting dual (or tri) regulation made sure you didn't lose your noid if one of the regs failed. But reg failure is less frequent these days (especially on guns with filters) so it's not as much of an issue anymore.
The main reason for using the pancake noid in the Rail is that Dye has become cheap. I doubt the valve's proof pressure is any better than an OTS solution.
For twiddlers, you can tune your gun, balancing LPR pressure and dwell to adjust the feel, consistency, and efficiency...to some degree. As you saw with your Shockers, the incremental change might not be enough for you to feel like the added expense is worth it for a new DM. That's a question of personal preference. (Although with DM and PM prices in the used market being what they are, I don't see a reason to buy either variety of Rail, unless you absolutely must have a new gun.)
Used Shockers are damn cheap too, and the lack of LPR means one less thing to go wrong.