I dunno where I read it, but I also remember I read something like it's more for HD content and not for streaming. but Wikipedia says something different:from http://en.wikipedia.org/wiki/Dirac_%28codec%29Dirac format aims to provide high-quality video compression from web video up to ultra HD and beyond, and as such competes with existing formats such as H.264 and VC-1.
Maybe you were searching for Dirac in a Matroska (MKV) container, and there you found that the .ogm container is better for streaming with low bit rates than .mkv? http://matroska.org/technical/guides/faq/index.html (last line).
Maybe apart from the fact that dirac is not widely supported now, it could be the alternative to theora? It has most probably no patent issues and the quality is very good. Actually perfect for a free and high quality web video standard.