Sometimes you want to see what some specific person has committed.
Even if this person has a repo for his own, it is not easy to see as usually changesets from main repo are regularly pulled into that repo.
With hgweb, this is possible by just searching for the name, but the output is somehow limited, so it is not useful to see ALL commits by that person.
Also, maybe one wants to search for some specific other changeset metadata, so maybe just index them all. :)
Would be VERY useful for summer of code. :)