Don't think that's true. There are plenty of projects who only
accept bugs through bugzilla (mozilla, various distributions, etc.) and I don't see any evidence of your claim being true.
That explains why my bugs don't get looked at for months if
not years when I submit them to such projects.
I reported a bug that eats people's hard disks due to a bug
in the X.ORG PCI support code on sparc, NOBODY has fixed
the bug in 2 years even though a full bugzilla entry with
even a full patch fix is in there.
Bugzilla sucks, emails rules because it is in your face and
gets people to work on things.