So it seems that the Dubai metro purple line is still part of the plans for the metro in the long term. The purple line was in the master plans at the beginning but was put on hold when the financial crisis hit. But Matthar al Tayer is quoted as saying that the purple line is still in the plans for the next 5-10 years.
The purple line was due to run from Dubai International Airport to Maktoum International Airport along the route of the Al Khail Road. With the airport being delayed it was inevitable that the purple line would also be delayed but hopefully recent slow progress will continue to bring it back into play.