The DZ's don't need them do they? Although I think the Matts were using a header tank in their Smaragds in Cork. I think the are also in the pictures of the Cyclone on www.plprod.com , although I am not sure. The main pro that I know of is that you can put the tank of an un-pumped engine on the CoG, or in a strange position, although I am not sure. I am sure though for a competition plane, it would weigh a tiny little bit more than a standard set-up. I really don't know if a DZ would need one, but I really doubt it.
My $2.....
Robert.

Robert.