While it's easy to sometimes confuse the LRF with a court of law, it isn't (really). The objective at this point is to prevail in the court of public opinion. Most of the public find the concept of running 26 miles alien. Their is likely little appreciation for the world of difference between running '6:52 pace' and '6:00' pace, So the notion that someone might start off a marathon at the former (as Frank claims to have done for the first half of CIM), and then after 20 miles shift to the latter for the final 10k, might seem totally plausible (which of course it is not). For the general public, the notion that someone--over the course of 26 miles, might stop and stretch, go to the bathroom, have a sandwich or even take a little nap might seem totally reasonable.
For this audience, for which the detailed 'pace facts' are little understood, it may be more compelling to demonstrate that FM (or any runner of his alleged caliber), could put this whole issue to rest tomorrow, by simply getting some of 'the boys' to join him at the track and record a 5m tempo run at 6:20 pace.
We agree with you that such a test is unnecessary given overwhelming data demonstrating that FM did not and could not complete the LAM in the time he is claiming.
However, presenting this readily available option might help persuade the lay-public that FM must have cheated if he is unwilling to take up this very reasonable challenge.