I agree whitespace is hugely important, i've actually bought and read Tufte's books ;). I was specific about gtktreeview for a reason. Framing content with whitespace is a tradeoff.
Remember, not all widgets are equal. Pixel waste when showing a settings dialog is rarely important, you want clarity and the minimum number of choices possible. Whitespace here has almost zero cost.
Whitespace when showing a list of data has a very high cost. You can show less rows in the same screen, it's an order of magnitude easier to compare data when scrolling is not required. If whitespace were free in this context, gtktreeview rows in rhythmbox/banshee should have no borders at all, we could just increase whitespace to frame each row.
It's not illegal to use 'ink' to frame content if it results in a better ratio of information. Scrolling/sliding windows are a special case because a smaller row hight allows you to present more data, without scrolling.
Also about the HIG.
I think it's unfair to call it flawed because a (min. 10 year old) documents states something in pixels. Pixels are the mechanism, not the goal. It's pretty easy to rephrase the points in.. say em. Second the gnome HIG is for the desktop. Not phones.
I don't see laptop screens changing DPI soon (ie 250+). The work is just too high. Even the traces of resolution independence in OS X 10.6 were lower than in 10.5 (arstechnica). Of course if somebody invents the dual mode DPI screen as way to go forward we might get there. It is different for phones, new platform no legacy and all that. But high DPI would certainly be nice.
I don't believe in porting the same app from desktop to android/iphone without changing the layout anyway.
Lastly. davidz has actually tried to get a resolution independent GTK (although his works dates back to 2008, and danni did pick up the branch, but it seems dead). That's the actual hard work. Not changing the HIG - it just represents what's possible. Cairo was also required work, and was required work before gtk could be changed sensibly.
But i sense this is 1) getting off topic 2) Nitpicking on gtk.