That's what's going on in the other thread where some people are criticising Linus and some others say that he's not paid by them, so shut up.

My opinion is that even you're a volunteer, you should listen to criticism or don't call yourself a volunteer (as this usually means that you want to achieve something for the benefit of others (at least in part), or you're doing something in cooperation with others). If you're not completely isolated then your actions have an effect on others so you should be prepared for criticism.

Similar to the position some have that if you comment on a bug/flaw/shortcoming of an open source app, that you are somehow obliged to shut up and get involved in the bug report process. In case it's not obvious, I don't subscribe to this view.

I don't think it's valid criticism in the first place.

The kernel is only a single part of a much bigger "Linux desktop distribution". For desktop users, work on good default configurations for all software, X (or its replacements), window managers, package managers, backports of security fixes to old software, and many other things that Canonical does are just as valuable as improving the kernel itself.