I'm pretty sure the Gnome Guidelines prohibit or at least discourage the use of markup in the status bar.
I know. A really stupid limitation IMHO. I even had to replace gtk_statusbar by gtk_label to enable this. An added bonus is that this got rid of the ugly statusbar frame :)
It also risks creating a lot of extra work adding extra markup to the other status bar messages.
Don't worry, I already did this work :) Now most messages have this (where this makes sense, of course).
The statusbar can and will be disabled so it should not be used for any really important information.
It's not used for any "really important" information. The only change is that it now displays more helpful tips than before, and does it in a more accessible and easy-to-scan way. You can still work without the statusbar enabled if you know Inkscape well enough.
Essential information that should not be ignored belongs in a Message Dialog.
Sure.