This thread has certainly provided some interesting reading, and I think that the original question has definitely been answered. As someone who is not GUE/DIR trained all I have is an understanding of DIR from the reading I have done (GUE's website,
Doing it Right: The Fundamentals of Better Diving and
Getting Clear on the Basics: The Fundamentals of Technical Diving) and from talking to more experienced DIR divers. I debated whether to respond or not, but I think some things need more emphasis.
Personally, I got tired of replacing 6" hoses that were weather checking from the tight bends. They were at best a crutch but an acceptable one as they were the best option available at the time.
In addition, a 2" or 2.5" SPG on a short hose presents several line traps and is generally more exposed and prone to impact.
In the same fashion every regulator hose, every stage/deco bottle, and every little thing exposed is a line trap and entanglement hazard. Should you seek to minimize those hazards? Of course, but at the same time if that line is going to catch your SPG hose, then it's probably also going to catch the regulator hose, and it's not like a line couldn't get caught around the button gauge. I see it this way, my 7' long hose is more of an entanglement hazard than my old short hose, but the potential benefits of the switch should I need to share air are greater than the added risk. In the same sense the benefits of an SPG on a hose secured to a stage/deco bottle outweigh the reduction in risk associated with using a button gauge (if it can be considered a reduction in the first place).
In the past I agreed the smallish 1/2" button gauges were indeed too small to read easily under water. However the current crop of 1" button gauges are quite easy to read under water - and I have 43 year old eyes. I suspect the younger DIR divers can see them at least as well. You can still see them just fine when you breathe down the reg during the gas switch.
Whether your particular reg points the gauge in the right direction is another issue. Personally I have no problems with the standard Mk 5/10/20/25 layout.
As others have mentioned, it's not just if you can see the gauge. DIR diving has a strong emphasis on buddy skills and team diving. Button gauges both reduce the accuracy of the reading (at least now they do) and make it harder for your buddy to see your gauges. It's easier for someone to turn an SPG on a hose to look at the pressure, then it is for them to orient themselves to be in the more limited field of view of the button gauge.
In my opinion the functions of deco regs and stage regs do differ in terms of both depth and mixes used. Can one reg do both? Yes. Is it optimum? No.
Every regulator has its own limitations of course. But from most of the reading into DIR I've done it seems that if you dedicate regulators to specific mixes/tanks, then the feeling is you'll be more likely to switch to the wrong gas at depth by using the regulator as identification rather than the tank markings. For example if your deco mixes are all in 40 cf AL tanks painted the same color and you accidentally place the regulator you "always" place on the deepest mix on your oxygen bottle, then switching to your deep mix regulator at depth could cause you to unknowingly switch to Oxygen. So is dedicating your regulators to individual mixes optimal? For those who dive DIR the risk of misidentifying your stage/deco at depth makes it less optimal than having regulators that can be switched between bottles without worry.
Also, I am suggesting that all SPG's are potentially innaccurate. What I am suggesting in terms of pre-dive checks is to check the deco tank(s) pressure(s) with a dedicated and known to be accurate pressure gauge (not an SPG) prior to the dive.
Everyone should be checking tank pressures and verifying mixtures prior to the dive. That is certainly agreed upon.
What I am also suggesting is that we actually advance our configurations and procedures as equipment evolves rather than blindly doing only what was done in the past - which apparently means I am not DIR enough to satisfy those DIR divers who are thoroughly (overly?)indoctrinated.
Until button gauges deliver readings to the 100 psi, and until it's not harder for the team divers to read that gauge than it's not a step forward and it's not advancing the configuration.
As I stated at the beginning of this post, I'm not DIR trained. I've just done my research on the configuration in part of my preparation for training. If any of the things I've said aren't DIR or contradict DIR principles then please point them out, but my statements above reflect my understanding of DIR diving at this time.
-Dave