JSON is very good, no doubt about that - but i think that even JSON can be optimized, does not have to be by MsgPack, but probably by a binary format.
As to text vs binary, I'll just leave this link here: http://catb.org/~esr/writings/taoup/html/textualitychapter.h...
i guess there are pros and cons on both sides, certainly nice would be something like a native BSON implementation with extensions in the browser/debugger to make it readable
When MsgPack was written a lot of JSON in the browser was still not native so an argument could be made if you desperately needed that bandwidth and it worked better than average for your specific workload. That is simply not true anymore.
Speed in the user's browser is VERY important.