Aside from "matching", given that this is the rear wheel, wouldn't a wider rim provide a smoother ride, especially right under your butt?
Some people claim that but I haven't noticed it with my XC279 wheels. I think you'd have to run significantly lower tire pressures on those wider rims in order to get a smoother ride. The wider rim alone doesn't do it.
By matching I don't mean looks, I mean tire profile, which would affect handling. I've run a wide rear and regular width front and it seemed ok to me but someone else might think differently.
Yeah, the tire pressure would need to be lower, but I'm not sure you'd need to go too low either. I swapped a 19mm rim for a the XC279 at the front, dropped the pressure from 110 to 95 and have noticed some improvement in comfort, using the 25mm GP4000s. I'm running a 20.5mm DT Swiss RR1.1 in the back with no handling ill effects, FWIW.