I have always thought that it is important to really know your airplane. Going way outside normal operating angles is just one invaluable piece of practice.
It gives a huge amount of confidence when being thrashed in a storm for example.
A roll in a 707 is a non event, but I'm really surprised that Johnson did a chandelle. I always found it very difficult to maintain less than 1.5 g on this maneuver, and almost always nearing 2g.
The real problem is that so often the second manouevre of a string goes pear-shaped. The temptation to put an even sightly stressed airplane back in the hangar--and say nothing, is too great. As said above, ready for some unsuspecting colleague to take it over.