I am working on the same question. It seems that it's most worth it if:

-You plan to fly First or Business Class
-The value you place on flying F or J outweighs the $750 to $1000 in taxes and fuel charges per pax required (seems it does for most on these boards)
-You don't wish to book and ITN with very many stopovers (since all legs must be on BA metal and therefore connect through LHR)
-An Open Jaw ITN is valuable to you.

In your case, booking two tickets in F/J from JFK-LHR-DAR would be a pretty amazing value even with the fees.

In my case, I was hoping to use the voucher for a month long, multi-stop jaunt around Eastern/Central/Western Europe, which appears to require connections in LHR each time. It could still be useful using rail travel interspersed.

Additionally, if you plan to use the voucher next summer, be prepared for July and August to be completely blacked out due to the Olympics.