That's true - but a heavy barrel also handles slower (more momentum to overcome) and causes fatigue faster when held on target or at the ready for longer periods.
IMO, the thing to look at is the expected (note, that's expected) use. Let's say you're using GI or similar ammo, and position shooting. Right there, we're lucky to have a 2 MOA standard at any realistic distance - more than likely at mid range we're closer to 4 MOA (keeping rounds on a siloughette -but that's about all- at 600).
Would the difference between a heavy and a light barrel be noticable in that circumstance? Nah.
Now let's say you're going to be using match grade ammo and shooting from your ruck or bipod. NOW you're going to be running much tighter groups, and the heavy barrel might be worth it. But the next question would be: why would you be doing so with a carbine?
If you want to run an XCR in the SPR role with magnified optics, I can understand the heavy barrel. If you're going to run it with an Aimpoint / EoTech / etc mostly from offhand or position shooting with a sling, the heavy barrel just doesn't matter.
FWIW, the one I just ordered ALSO has a light barrel.
To make things a bit more complex, I don't believe the barrel that came on my others (light barrel, 16") really is a light contour. I believe it's more of a sporter contour - a touch heavier than say the pencil thin barrels of the Colt SP1 and such. And, in all honesty, a touch heavier than I'd prefer in the XCR. So by my reasoning, the XCR "lightweight" barrel is really closer to a "government weight" or "sporter weight". Inbetween. As that's a touch heavier than I prefer for a carbine, that's what I stick with ;-)
As always, YMMV.