There is a reason Stoner designed it with a 20" barrel to begin with... that round relies on velocity for its lethality. They are not very ballistically effective after the bullet drops below a certain velocity, and velocity really starts to suffer after you go shorter than 20". If all you ever use it for is close quarters, then a carbine is fine... but for my purposes (an all-around battle rifle) I prefer the 20". As has been mentioned, sight radius is a plus too; and so is the longer handguard for some positions I use.
I have never really felt that a 20" rifle is particularly unwieldy or overly long... I still consider it a carbine as compared to something like a .30 cal battle rifle, which is what I mostly shoot. As long as it has a lighter-weight barrel like an A1 style or A2 gov't profile, it is plenty maneuverable. Then again I don't spend any time in kill houses or kicking in doors either. But if I did I would have a carbine dedicated specifically for that purpose. I think a 20" is a better all-around rifle for dealing with 2-legged varmints at any practical distance.
I am not really caught up in all the nostalgia so I don't go for the A1 clones... I really like the innovations of the flat top receiver, the collapsible stock, the modular handguard, and improved magazines. I also like the faster twist rate so as to be able to shoot heavier bullets and extend the max effective range even more.
My next AR build is going to be a 20" GI-profile 1/7 twist barrel with a flat top, CTR stock, MI-SS 12" free float tube, and fixed front sight, all from PSA. I'm just waiting on the lower. Top her off with a TA-11 ACOG, a KAC flip up rear sight, and a tactical sling of my own design that can also be used as a loop sling for precision use, and she will be ready to do some damage.