Maybe if they were from a friend's feed and they wouldn't email me the originals for some reason or for convenience... but when they are my own pictures and I have the originals... doesn't make sense to me.
The older photos on Facebook are pretty low res, but they do now have higher res ones available and are the proper res for 4x6 prints, I believe they have had this since adding the full screen image viewer.
I very rarely print stuff these days but I can understand the convenience factor - I'm probably not your ideal demographic anyway since I very rarely print, it would not be that that inconvenient for me. Plus I only have one friend :(
Actually, the image quality you get from FB through their API is dependent on both the photo size that is being uploaded, and the quality you ask facebook to save your photo at. A high resolution photo that is uploaded to Facebook in high quality mode is a good photo for a 10x15cm print. Any bigger print size and you start running into resolution issues.
The trick here is to show the user which photos he will have low resolution issues, and which he won't.