I'm not sure I understand the attraction. First, getting an "upper" anything simply gives you one and a half guns for almost the same price. Why not just buy a second rifle? Second, if I'm considering a heavier caliber, magazine compatibility, though attractive, would not be the selling point for me. Ammo availability would. The ballistics of the hard to find 300 are similar to the 308 (7.62), which are available everywhere, so why not just pick yourself up a complete S&W AR10 ?
As I am in the process of getting a 300BLK SBR upper and I own a S&W M&P VTAC II, I can tell you at least *my* reasons.
1) SBR with subsonic or supersonic ammo. Supersonic good to 300+ yards, although ppl have 500, 750, and 1000 yard consistent shots with a SBR.
2) Sub MOA from a 8" barrel (potential CQC/HD applications)
3) My lower is already configured nicely. Sure, a new lower could be configured in a similar manner.
4) .380/7.62 casing has a larger taper, and has been known to cause feeding issues with standard PMAGs. Requires frankenmags and a different BCG. Technically all you need a is a new barrel for the 300 BLK.
5) 300BLK ammo is getting easier to find; also, it's very popular for reloaders because it uses arguable the most common parent cartridge (.223) with the most common reloaded bullet (.308). It's only 3 years old, give it time! Maybe it will become a bit more popular and stocked in more places.
In short, it's a round designed to run in an SBR, suppressed with subs or supers with excellent terminal ballistics and penetration capabilities.
Sure, you could get a complete rifle for the cost of just the upper. Nothing wrong with that. Or you could go with an AR-10.
To each their own, though..