The DevTools features are not restricted to those special case things like the Audio editor, which probably only use a handful of people (and which would better be put into an add-on, IMO), but they also do some things better than Firebug like the console message filtering for example.
If you followed the reports on Bugzilla and publications by the DevTools team, you'd see that they do look at Firebug (and other tools) while creating their features. There are other reasons why they didn't just integrate Firebug into Firefox. So, please don't claim things when you don't have any knowledge about the background!
Anyway, fact is, Firebug 3 will be mostly a theme for the DevTools besides adding a few features to them. Therefore, issues you see while using it, should also be checked when Firebug is disabled. If they still happen with the pure DevTools, you should open a bug report for them in
Bugzilla, otherwise open the report on
Firebug's issue tracker. If you don't do that, there's little chance those issues get recognized and fixed.
Sebastian