If you look at my thread lower down you'll find it seems to be quite common. I think some people get lucky but on the whole purple engrams rarely give a legendary item (I've had one in ten). Dunno where you've been farming for them but I've found that barring doing the exploits that involve dying repeatedly, the best spots to get purple engrams to drop are the well known ones on Earth and Venus. The side effect is that you'll get plenty of blues to decrypt giving you a higher cryptarch rank and with it the blue and purple freebies he then sends you.