Here's the FAQ on Black Bars. There are lots of different aspect ratios and black bars are a fact of life if you watch your channels OAR, unstretched, original format, etc.

http://www.digitalhome.ca/forum/showthread.php?t=76089

Most product produced for TV was 4:3 and is now 16:9, however, movies can have any aspect from about 1.1:1 to 2.7:1. This is the director's prerogative, as you mentioned, but there can be directors of TV programming too, that may want to "stand out", although for most TV I haven't noticed much programming outside of 4:3 or 16:9..