Is there really a need for something between the 5.56 and the 7.62?
Yes, and using current doctrine and practice, this decision does have sound, if academic, principles behind it.
Whether this will be borne out in actual practice will remain to be seen.
Whether we wind up in a situation where the presumptions are carried all the way out to the planned extremes, also will remain to be seen, as well. (I'd rather not get into enough shooting war to test our TraDoc to its core.)
The tactical principle can be described simply enough. We have to skim over the complexities of combined arms operations and minimum maneuver unit sizes (that's a thorny debate for other fora).
But basically we are addressing the specific needs of a Rifle Company in contact. A Rifle Company applies itself to the task by deploying its Platoons, which, in their turn, apply their Squads (Squad, Platoon, & Company size & organization are a bitter and acrimonious debate that is over a century old, and still not resolved, and is also best argued elsewhere). The Rifle Company has a dedicated Weapons Platoon to support the Company's operations. Those operations will support the Platoons. Which leaves the Squads some what on their own, unless they are being specifically supported by Company (or Higher) fires. Which means the Squad only has their own rifles to achieve their immediate goals. They are reliant on being able to call upon Platoon or Company assets in support of either offense or defense. Which gets complicated. It's very common for a Company to only have two GPMG they can use, which might have to cover 9 different Squads from the 3 Platoons.
The 5.56x45 Rifle ammo gives a reasonable reach for any one rifleman, and good utility for a Squad of them, but, by definition, "rifle range" works both ways. If the Squad is in offense, it would be nice to be able to suppress the enemy at, say 200m or 400m beyond "rifle" range, especially with heavier projectiles (there's still another LMG argument, about half a century old, that is also not germane here). In Defense, the Squad also wants to be able to reach out beyond rifle range to suppress or redirect attacks.
The current answer is to either put a lot of 5.56 ball out there using a SAW (like the M249), or to embed 7.62x51 support at some approximation of the Squad level. And, that 7.62x51 is really optimized for Platoon or Company level use, rather than the Squad level.
Enter in this new 7x51 (or 6.8x51 or .270"super" or whatever. More reach than 5.56, less reach (if quibbling for now) than 7.62x51.
The Army has already stated that they have no intention of replacing infantry rifles in any way, shape, or form. And, that's logical in the same way that not giving every infantryman an M240 is logical. Or in giving them all an M247.
The SAW gunner is a person already humping a heavy load. It's a relatively unique load, too. So, changing the ammo probably will not have much impact on the logistics (back injuries, maybe--again, different topic).
Now, DoD has always wanted--as they always do--this new round to be everything, better than first bicycles, sliced bread or Buck Roger's Ray gun. This new round probably would work just as well as 7x45, but it's meant, by DoD, to be capable of killing tanks, anti-satellite operations, and defenestrating Ming the Merciless--roles far beyond the needs of ordinary grunts in a Rifle Squad.
Will it wander out into the civilian word? What hasn't?
Will it have a use? Dunno, probably only to further complicate arguments over .270 vs. .280 vs .243 (or any of the .25's). Does not take a crystal ball to assert that an AR upper of some sort will show up. And that there might be a bolt action or six (and that Kel Tec will probably beat the market).
