Valid, in both contexts put forth here mean something that works from a basic starting point, and evolves. But regardless of how a defensive technique works, the first thing it must do, and do every time, to be considered valid, is neutralize or diminish the threat, period. This is the starting foundation of a defensive technique.
It apples to all defensive training. If a technique or system does not do this from the onset, with the most efficient movements, that can quickly evolve from defense to offense, it is not valid.