I think the side profile aesthetic (the standard manufacturer's catalog picture) was perhaps an influence as to why they weren't more popular. Kinda looks like that character from the Mystery Science Theater 3000 show. The one that sat on the right?
I do have to get me a bike with them. I pretty much always shift my dt shifter "racy" bikes both derailleurs at a time with one hand. It seems like it'd be really easy with this setup.