I still use FS9 alongside FSX-SE. Two main reasons, first are the Dreamfleet 737 and RFP747. I love those and they are not available on FSX
I use 3D panels only aircrafts in FSX thanks to Chaseplane and this is quite great. But, with the aircrafts that do not have a great 3D panel (MD11, 767LDS...), I prefer using them on FS9 with a 2D cockpit. I do not mind the lower graphic quality (IFR flights with airliners) and I prefer the way 2D cockpits are managed with in FS9 (especially when it comes to looking down on landings).
Also, I really like FS9, I do not know why, but each time I go back to it I really enjoy the experience.