I don't really see the point myself.
I would rather have completely seperate whole guns.
For example:
I have an SBR with a 7 1/2" barrel with an entry stock. I have an SBR with a 11 1/2" barrel and a Sully stock.
I have an SBR in 9mm with a 11 1/2" upper and a Sully stock. I also have an LRM M169 Suppressed 9mm upper.
I am going to by a Colt RR if the guy will call me on the phone so I know where to send the money.
If/When I get that, I am putting a VLTOR stock on it.
I can switch any one of those uppers to any one of those lowers. I can not only choose the barrel length, I can also choose the stock. Since they are all flat top receivers, I can also choose the optic: I have the Aimpoint ML2 and 3. A TA31F Acog. And a 10X mil dot scope all in LaRue mounts. I can put any optic on any upper. All uppers have BUIS.
Of course I can also run any of the 5.56 lowers with 16" uppers or my 24" scoped upper.
All the uppers except the 7.5" are equipped with the AAC flash suppressor so I can use my AAC M4-2000 Suppressor on any of them.
I can also use the various vertical foregrips including the 900 series Surefire weapons light with a LaRue mount or the bipod on a LaRue mount on any of the uppers because they all have a rail system.
The point to all that is to illustate that the AR15 system is very modular to begin with. You can very easily change configurations as is: I don't think that being able to change barrels on one gun would add anything to the mix.
That MGI thing is cool, but it isnt' select fire.