I don't normally go for single regging myself - I prefer the safety buffer of dual regs - but I didn't have any problem when I tried it. It didn't give me any real benefit though - consistency was about the same & there was no real weight/profile benefit - so I didn't stick with it.
As for the usefulness of the precision. With some of the mini-gauges out there, you have shocking levels of inaccuracy at times - I had one on a 'geddon that was about 200 psi out when I spotted the problem!
OK, AIRs can go faulty & misread, but that's a relatively uncommon fault (now awaits hundreds of people piping up to say that theirs went wrong
)
Plus it's nice to dial straight to your desired output, with the fudge factor on mini-gauges, you sometimes have to P about tweaking the output. If you have a problem where you're not sure if it's the air system, your secondary reg, LPR, whatever, it's a nice comfort to know precisely what one system is doing.