Synthesized font faces... Yes, there is value IMHO. So, while I'd throw out a vote in favor of getting Cairo to show it on canvas that is because of the following...
There was a discussion a long time ago about our use of synthesized font faces and it went into territory that I still think no other graphics app has gone into yet (that may have changed over the years). The eventual goal was to do more in-depth synthesizing. Effectively, allow for more/less bold or slant rather than what it currently is (single, fixed amounts). In other words, (if I'm recalling correctly) the original idea was buttons for bolder and lighter which would continue to work, not just be toggles.
The best I can find is a message referencing the other discussion.
http://sourceforge.net/mailarchive/message.php?msg_id=19520114Regarding Alexander's desire to see something other than just buttons, one could probably use spinbuttons or sliders given that there should probably be artificial limits imposed because the functions get worthless after a certain point.
Before we get into discussions about room in the text tool controls bar, it's all the more reason to follow GIMP's lead with floating controls on canvas for the primary features of the tool. And yes, I'm going to bring this up every chance I get until someone gets annoyed enough to implement it. :D