D-Plus

While glancing over the Entertainment Weekly movie scorecard (basically, it rounds up a select few films now playing in theaters in a grid and shows us the letter rating they received from various critics around the country) I found myself fascinated by the D+ rating.  Seriously, what's the point of a D+?  What could possibly make a movie a D+ rather than a flat out D?  To me, once you assign a movie a D rating of any stripe, you've pretty much said the movie is awful.  A D+ does not soften that blow.  "Wow, that movie was horrible, but not quite D level bad- I'm going to go, eh, D+."