Thanks for reverting Virinchi ji. No i dont doubt why VD is used in general but I am sure there is purpose for other general dasa system like CK Dasa, Moola Dasa, Kalachakra Dasa etc exists and been taught Sage Parashar .
I do agree most of the people use VD by default and reasons could be as simple as they are only aware of VD from moon, to the reasons as most of the literature is available on VD and other dasas needs more experiment and R&D to be understood.
Also there are times when VD does go haywired and i am sure we can put blame on the astrologer's understanding about VD and his perception, but it might be justified also since there is no other way for an astrologer to cross check the even other than transit.
As an example:-Since Moon is mind and thought, what would happen if person gets married without being emotionally and mindfully involved in it. Will it show and how would it show.
Or in cases where person is actually emotionally and mindfully involved to his/her partner but didnt get marry to him/her in those years. How can we understand marriage in those cases especially considering VD is from moon. In those cases wouldn't other Dasa systems would help to reach the event.
Regarding Moola Dasa, I would defintely not depend upon D-60 for any prediction. It would be good in theory but considering its lagna lasts only for 2 mins, it would be really difficult to fix the lagna with 2 mins of variation.