Well, when you make a Drawer, you pass an options in, and the purpose of the $.extend is to apply anything from that options to the Drawer, so if it weren't for this line, you could still have useCanvas set by the options. 

Probably not very useful, though, so maybe we should just leave it as-is. If we ever want to add it explicitly later, we can. It's easier to add later than to remove later.