The only disadvantage is to US carriers at BFS
Jamie, it is a disadvantage to NI economy if their local airport is losing passengers to a Dublin flight to the US so much so that CO were questioning their route from Newark. In addition, although at this point in time unlikely that Aer Lingus would look at a transatlantic offering from BFS, with a taxation/ penalty on locals flying from Belfast, that may be just another reason why they might delay any potential route.
If we dont get a concession on taxation and create a more level playing field (with ROI) then CO may not stick around and we will in NI will have no choice but to go to DUB or London etc, and jobs lost etc.
People who want to go to the UK are not going to go to DUB just to save a few pounds
I also think if you had a family coming across from the US family (so say 2 adults and 2 children) an extra £60 each is substantial, so I could well see why they might choose to fly into Dublin!!