For this particular mission, probably not; rule of cool probably played a factor. (They also have a much-upgraded 1950s B-57 for similar purposes; https://airbornescience.nasa.gov/aircraft/WB-57_-_JSC) For other missions, though, being able to get up to Mach 1.8 is probably helpful when you might be intercepting a Space Shuttle on the tail end of re-entry. Manoeuvrability is also a factor; being nimble helps get around a test aircraft. Or chasing one of these: https://en.wikipedia.org/wiki/Lockheed_Martin_X-59_Quesst
