Vidyo uses an implementation of H.264 SVC, I believe. But in general it's not been widely adopted. SVC generally has worse compression than normal H.264, so for streaming it makes more sense just to store multiple encodes at different qualities.
Yes, SVC's sweet spot is more geared towards real-time than pre-cacheable streams.
SVC stands for "scalable video coding", and generally behaves in at least one of two dimensions: temporal or spatial.
Temporal SVC is pretty much what we have with H.264 AVC; it provides a means to support multiple receiver frame rates from a single sender frame rate. So, if Alice transmits at 60fps, a temporally-smart coding will allow a switch between Alice and Bob to drop some frames so Bob only gets 30fps, say, while Charlie gets the full 60fps. This is useful if Alice has a wonderful upstream connection and Charlie has a great downstream connection, but Bob is connecting via a crappy mobile connection.
Spatial SVC is the same thing, but cast in terms of resolution. Imagine that Alice is transmitting 1080p, and Bob is still on that bad mobile network. An SVC coding would mean that Alice could transmit a single high-res stream, and a smart media switch could send Bob a "down-sampled" 380p stream without having to decode and re-encode, while still sending the full 1080p to Charlie.
So. Clearly, this is awesome when dealing with real-time coding (e.g., video calling). But when streaming content from Netflix or Youtube or whatever, it's trivial to provide different streams at different resolutions to different users, based on prevailing network conditions for each user. The up-front cost of encoding and storing a dozen so different versions of the same content is minimal, and the work to identify the right stream for each user is about the same in either case.
Even for real-time use cases, the range of bitrates where (spatial) SVC provides an advantage over simulcast is relatively small, and it's not a big advantage. Plus, it's a patent minefield. I've also seen people argue that it has a computational complexity advantage over coding multiple streams, but a properly written simulcast encoder can share a bunch of the work between encodes as well (libvpx has an implementation of this for VP8, for example, for use in WebRTC).
Agreed; the way the numbers work out today, the advantages are minimal. The story looks like it might be a bit different with 4k on the horizon, though.
