The idea has some merit...
But I would rather spend half my money on a real rifle round and the other half on pistol rounds, then everything on pistol rounds.
I dont see the point in limiting yourself to the ballistics offered by a carbine when you could have a rifle, just for ammo compatibility. I really dont see a lot of situations where compatibility would offer enough to cut yourself down to the carbine level.
Now, personally I like the M-1 carbine- I dont have anything against them. They do have advantages. BUT I wouldnt recommend using a carbine over a rifle just because of ammo compatability. If you were going to use a carbine because it is lighter, or has lighter recoil or whatever, then by all means, do it.
Just dont use a carbine for the single point that your pistol uses that same ammo.