Yes, at least having the option is nice, and makes the rifle more versatile.I hear "mil-spec" in relation to firearms and I get nervous! Let me tell you some stories from my small-arms repairman days on what passes for mil-spec! But you're right 1/9 shoots just fine for most bullets. The TAP T2 bullets I have for a rainy day shoot just fine from the 1/7 and 1/8 AR's I have but leave much to be desired from my XCR. (However, a plain 75gr OTM bullet shoots OK, not stellar, but OK.) For plinking, I could really care less but I'd like to have the option/ability to use whichever.
I vaguely remember that being the twist rate needed to stabilize a .223 bullet that would be subsonic with a normal powder load (something approaching a 300gr bullet). Not sure if it has actually been done....1:4, that would be a really long bullet and reallly crappy barrel life!