A great idea, never really took off in mass production, presumably as their buyers didn't know how to fit a spoke or true a wheel, so it had no attraction as a feature.
I've not seen it on a mountainbike before.
Turns up on some classic tourers from Fuji, Novara, Raleigh Randonneur and newer Sojourn, and others.
Now on bijoux high end US tourers: Sacha White, Soma and Carl Strong at least. It lends that 'really though out' feel to a custom frame.
Geekhouse, Davidson and Surly (LHT) bizarrely fit them on the LH chainstay; totally missing the point. :?
It's certainly mentioned as a design feature in the mid-80s
Paterek framebuilding manual, and I think it turns up in
Tony Oliver's "Touring Bikes" book.
but are there other downsides?
-Different spoke length needed if you change your hub flange size, but you can fit extending spacers for longer spokes.
-Make sure the nipples are done up tight, a stray spoke could do some damage.
-Tiny extra risk of chainstay cracking at the BB braze-on end.
Oh, and other bike people stopping you to have a look
All the best