Hmmm.. "Behave properly"?! That varies inside Inkscape itself, therefore I consider this to be Inkscape bug...
Agreed, I should have said Inkscape doesn't behave properly. For example with Gill Sans if I select the Medium style the font-weight gets set to bold. Similarly, if I select the Book style Condensed actually gets used. In my case, using the toolbar has the same, incorrect, effect as using the font dialog. Sorry I wasn't more clear earlier.
Yes, there's been a bug about this around for a while. Right now Inkscape only really undestands 4 variants. Everything else is matched to those in some way.
Pango and fontconfig now support preferred family/style names so a large family can be under one single name with many variants. But Pango has a bug with one of the 10 possible weights, also there's an issue with exotic variants if they share width and stretch with a more generic variant. You can see that with Thin with ExtraLight, and Contour with Regular.
In a perfect world, users should see one family name, with each variants being listed and properly selectable and applicable. This means throwing the current 4 variant limit code from Inkscape and relying on Pango for the rest. That would already fix most of it. Then the rest is a Pango bug.
Cheers,
Denis Moyogo Jacquerye