With the benefit of time, I see that Phil is absolutely right. Today writing a printer driver that posts to a blog instead of printing is ‘good enough’. Soon, a Save to blog function powered by the MetaWeblog and ATOM APIs will be good enough.
PrintTo, on the other hand, is an OS service available to all applications, even ones that have no idea what printers you've installed, or what rendering engines they use (Postscript, HPGL, Clearview, etc.). So every application that can print in your OS should be able to write to the web.