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The ByteString interface was added to Web IDL in order to make it easier to explain legacy behavior in XMLHttpRequest. I'd like to ask that it be removed because this legacy behavior is not a good model for new APIs, but having ByteString in Web IDL may lead API designers to adopt this model. The legacy behavior in XMLHttpRequest can be explained in the XMLHttpRequest specification. This topic is being discussed on this thread: http://lists.w3.org/Archives/Public/public-script-coord/2013JulSep/thread.html#msg56
I'm opposed to this as should be clear from reading the thread.
After reading the thread, I think it's sufficient that there is a warning in the spec not to use ByteString unless there are clear reasons for doing so over DOMString.