No idea when they first started appearing, but you certainly see a lot of vintage English bikes with wrap arounds.
As far as advantages/disadvantages, I've haven't brazed any up that way (yet), but they'd be a bit more difficult as both seat stays have to meet up at the same exact place so they're even. Here's a modern, available wrap around casting (from framebuildersupply.com) that I'd probably use if I wanted to to do that.
A clear advantage would be more surface area for the braze filler material. More surface area = stronger joint.
If I were to braze seat stays on "uncoupled", I could be off by a bit side to side and it wouldn't matter structurally. Visually it would bother the heck out of me though.
