Actually, I love carbines in .357 and .44 Mag. From 16" barrels, then gain a lot of velocity. And you can have a common ammo for you sidearm.
Not sure about .45 ACP though, but yeah, if there is no gain from the barrel length and here in MA I am limited to a 10 round mag, I will take a pistol.
But it is still one neat firearm. Now NFA it and cut the barrel down and it might be even more interesting.
Sorry, I meant 'no benefit' in a semi auto rifle being pistol caliber vs. rifle caliber long gun. You do point out one valid bene however and that is the common ammo with your sidearm. I'll conceed that.