A manned capsule escape test is not the first (or third) thing they would launch on NG.
By that time they should(tm) have one or more well used 1st stages. Then add a stock 2nd stage and do the test for real. Maybe do that launch partially fueled to keep the environmental impact small.
Blue is primarily interested in selling seats to tourists, they need to convince the public that their safety systems work. Easiest to do that using the actual flight configuration. For NASA they could go with a cobbled up rocket and try to save some money, but that is more difficult to explain. Also, once the booster is "free" the cost for such a test go down by a good margin. - If reuse does not work out Blue has a bigger problem.
Convincing the public is why I think that we may see another NS abort test in the future, once a booster and capsule combination reaches their end of life. Reinforcing the message that the safety systems work as intended over the full lifetime of the system.