I have just got a question about RNAV , we were told that if controller vector aircraft leave the RNAV ,we should tell pilot direct to a fix when we want aircraft back to the procedure rather than a heading,like "XXX,direct to AA111 ,join DOGAR7A" ,but I asked a pilot if he can join the procedure if I just give him a heading not a fix ,the answer was affirm and he really did it ,the point he intercept the procedure is not any fix of the procedure ,can anybody tell me why can not use a heading to intercept the procedure ?