Panning a scene with your viewfinder

Yeah, but keep in mind that Sony is catering to amateur video shooters, like moms and dads who want their DSLRs to act like camcorders. Running a flapping mirror would create noise that could get picked up by the onboard microphone (people are already raising a racket about lens focusing noise in movies). Not a problem in a full feature film recording setup with sound recording done separately, but quite an issue with moms trying to film their kids performing and getting extra camera sounds in the clip.