And single socket is equally cheap, except it takes twice the rack space - but it also gives you redundancy. One server can fail and you can carry on.
The advantage of memory bandwidth vs Ethernet for scaling to x2 really doesn't matter. If it did, you're not horizontally scalable and at best you buy a little time before you hit the wall.
If the price difference isn't much, I would heavily prefer single socket.
Dual socket is a cheap, easy, and common. If only to recycle fans, power supplies and racks, it seems useful.