For several reasons, actually. Firstly, people at my FLGS do not usually recognize non-GW rules. Secondly, "pretend aircav" lets me take more valks (130 pts per instead of 190), not to mention BS 4 vets as troops, astropaths and my favorite IG character, Straken.
I realize that squadding valks is not ideal, but it gives me the opportunity to field 6 of them, and it forces me to be fluffy in game situations (aircraft don't fly randomly about the battlefield, they deploy in groups).
"Employ your time in improving yourself by other men's writings, so that you shall gain easily what others have labored hard for."