Reader Point: It is highly unlikely that a martial artist testing through correspondence would ask another possibly higher ranking Martial Artist to test for them...what would that say about the other better Martial Artist? This may happen but I would think this scenario would be quite rare. Most respectable organizations as AIKIA have safe guards to such possibilities as in "proof of your driver’s license" with a picture and if it does not look like the guy on the video there would be an issue!
TKDTutor response: In-person, the testee must test at the allotted time, whether he or she feels like testing or not. Many times, they have traveled many miles with little sleep before a testing or may be injured, sick, or recovering. They have to have the resolve to perform at their best no matter their circumstances. Video testees can pick the time to perform the requirements to insure it is the opportune time for them to perform at their best.
An in-person testing is an event that must be experienced by all. There is no substitute for “live” participation and viewing. This why people pay so much money to see bits and pieces of sporting events in-person when they could see the entire event for free on television, with close-ups and continuous replays of all the action.