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Many people misconstrue evolutionary science to state that any mutation that is less clearly adaptive than another - however trivial and inconsequential to survival - will not perpetuate due to that initial reduction in fitness.

Survival of the adequate? Actually, I see your point. If no other mutations were to ever occur, the primates with the stronger jaws would probably outlast the primates with the weaker jaw, over a number of generations. However, since other mutations do occur, the primates with the weaker jaws are in a position to profit from them, in the genetic long-run.

One may also posit that since increased intelligence is such an obvious evolutionary advantage, that the primates with weaker jaws would swiftly begin evolving larger brains in relatively few generations time. Those weak-jawed primates born with larger brains would have a higher probability of surviving to reproduction than those weak-jawed primates with smaller brains (independent of the jaw-strength) and so once the genetic provision was made that allowed for larger brains, they would quickly expand to fill the available space and thus become the norm.

Survival of the adequate? Actually, I see your point. If no other mutations were to ever occur, the primates with the stronger jaws would probably outlast the primates with the weaker jaw, over a number of generations. However, since other mutations do occur, the primates with the weaker jaws are in a position to profit from them, in the genetic long-run.

Another way to think about it is that all sorts of mutations occur and have no effect on Dariwinian fitness of the species until the environment changes and the mutation then confers an evolutionary advantage on its holders. A good example of a mutation which is just there and has yet to confer an evolutionary advantage (or disadvantage) is the ability to roll one's tongue. Some people have it, other don't and it doesn't seem confer a any kind of evolutionary advantage either way. Until an environmental change happens that favors (or disfavors) individuals with that ability, it just continues along in the genome.

By the way, the loosed jaw mutation wouldn't necessarily confer an evolutionary disadvantage on individuals with it in an environment with abundent "soft" food. If there was already a genetic pre-disposition in primates for larger brain growth, the occurrence of the jaw loosening could have been the key that unlocked the door to bigger brains in a relatively short timespan. That, in turn, would have conferred a significant advantage on the proto-humans.