Yes there is a problem with using a receiver block. You can spin the barrel right out of the extension like the guy above did. People do it all the time and then they have to send it out to a gunsmith to have the extension torqued again.
I think you have a very good idea here. The AR is already susceptible to being tinkered with... you are not going to create any NEW issues with jackwagons fudging up rifles by doing stupid stuff. As a matter of fact, this idea would probably ELIMINATE some of the fudged stuff from happening (namely when people try to change muzzle devises without the right tools). The other people who have the proper tools and know-how will not experience any problems with the new barrel feature either.... I'd be willing to bet many of them would chuck their old tools and use the new feature.
You should consider contacting some of the more progressive aftermarket barrel manufacturers with this idea. Or start your own barrel company hehehehe.
I thought of this a LONG time ago.
Looks like Colt may have beaten you to the punch. Pic from article about Colt winning gov contract to update M4 barrels to M4A1 profile. Looks like the new barrel has flats on it. Attachment 24369
That is not a new barrel.
The vast, vast, vast majority of people don't go around swapping out muzzle devices all the time. There is little reason to completely change how we make barrels due to a tiny fraction of a fraction of the customer base.
For those folks who manage to torque the barrel out of the extension, they should take that as a good omen, since they were probably doing something wrong or the barrel wasn't built up correctly to begin with.