Just to add my two cents to the plea here's my use case.
Given the high usage of open source libraries in my job it happens quite often
to have to look to the some part of the source of them just to clarify their
usage or simply to borrow some implementation ideas.
Recently i was evaluating Netbeans and i needed to look to the j2ee.jboss4
source to see if i could expand that plugin for our needs.
Given that the downloadable source zip is corrupted (it contains all the project
directories but only a very small number of source files) i had to resort to
download from the mercurial repository.

Well i only needed to look into j2ee.jboss directory, downloadable size 1.04 MB,
size on disk 1.39 MB, no need to compile it or the whole Netbeans project, but
nonetheless  i had to clone the WHOLE Netbeans source, downloadable size 1.35
GB, size on disk 1.74 MB, about 6 hours of download time. 
Almost 1300% increase in size, download time, bandwidth occupation respect to
what would be needed if partial cloning was available, and bandwidth doesn't
come for free. :(

IMO this together with issue883 are serious show stoppers that don't allow
Mercurial to spread as it would deserve.