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).