That is a benefit right there. Instead of having to fiddle with multiple sticks, you have same performance/spec on one piece.
You're ignoring how extremly finnicky the upgrade process is. You're far better off selling your RAM and buying a new kit, than risking adding more sticks. Mixed kits are nothing but trouble
That's the key (supposed) benefit struck down. So it comes down to installing a 2 piece or 4 piece kit into a 4 slot "socket" for lack of a better word, versus installing a 1 piece kit into an area roughly the same size.
In addition, a 2 piece kit is prefered in almost all cases except for some AMD CPUs. Most of the time installing 4 sticks will require a more expensive motherboard or will be limited, speed-wise.
Further benefit of CAMM2 is air cooler compatibility.
There are many benefits for desktops if you think about it. And so far, no penalty aside possibly the area taken up by the RAM, and possible difficulty swapping kits.