I think the demands are different. A backgas reg will be used at depth where the gas is relatively thicker (unless you are using He mixes) and the flow rates are much higher. CO2 retention is also a risk factor so an easy breathing regulator is needed to minimize CO2 retention. On the bottom you are also potentially gpoing to be working/swimming harder which increases both flow rate and CO2 concerns. Temps on the bottom are often colder and combined with higher flow rates, better freeze protection is needed (at least in colder water) All of that points to the need for a very good performing first and second stage for your backgas.
Deco regs on the other hand are used in midwater or shallow water and are normally employed by divers who are stationary, hanging on a jon line etc, and are not usually working hard for sustained periods of time. Consequently flow rates are lower, CO2 production and retention are not an issue and not as much brute performance is needed.
Relaibility is however crucial. Having a backgas reg quit means switching to the secondary and aborting the dive with less than the expected deco time. Having a deco reg quit means loss of that deco gas (unless you can swap, and consequently flood, the regulator underwater.) That means a much longer deco schedule on back gas or one of you two originally planned deco gasses.
My preference is to use super simple deco regs that won't quit as they have few parts to break and have few o-rings exposed to high pressure air (which helps reliability and decreases the potential of an O2 fire.) Plus, the odds are better that you can flood a simple unbalanced flow by piston reg and get it to work successfully than they are that you can do that with a balanced diaphragm reg.
Another advantage in my opinion is the "warning" you get if you have a near empty deco bottle. At about 300 psi, the inhaltion effort goes up and lets you know you are just about done. Consequently, with a MK 2 on a deco bottle I only use a button gauge to confirm that it has about what it is supposed to have in it for pressure during the dive which avoides the extra bulk and potential entanglement and failure possibilities that accompany an SPG on a 7" hose.
With regard to ease of breathing, I do compromise a bit and use an adjustable second stage on my Mk 2's and 3's (either G250's or Balanced Adjustables). That lets you set the inhalation resistance at essentially the same level you would get with a high performance second stage, but then if you don't readjust for lower tank pressures, still gives you a near empty warning. It's a compromise however as it is a lot more expensive and an adjustable second stage is a bit more complex - in my opinion not enough to matter as they are still very reliable, but in theory they do have more ways to fail.
Even an R190 however can be tuned to breathe fairly easily (still not super smooth but with a low inhalation effort around 1.2" of water). So if the breathing performance is acceptable to the diver I see no problem in using an unbalanced second stage on a deco reg.
But that's just my opinion and I have to plead guilty to a 41 year history of being a very independent kind of person who wants to know "why?" and is genetically unable to follow a crowd just "because".
