This would actually be very useful. I've changed my view a bit since my comments in the other thread: zip.js works pretty nicely, but can often end up janking on synchronous manipulation of blobs (copying/slicing/creating etc). It also does not seem to be able to take advantage of multiple cores, only offloading work in to a single web worker (although I don't know how many compression algorithms can be effectively loaded across multiple cores). I think moving this in to the browser would make it easier to take advantage of in a fully asynchronous way.
I'd also point out I think the ability to compress and decompress data should be treated separately to the support for archive file formats, such as the .zip format. Both are useful and they are of course related, but compression of raw data doesn't obligate support for an archive format.