I don't think you should weight it like that. If you have multiple proposed syntaxes, and both are equally effective with code highlighting but one is moderately ineffective without highlighting, then it makes sense that you should go for the latter. The weighting controls how much of an effectivity loss in the code highlighting case you would be willing to trade off against effectivity improvements in the other case.
It sounds like you're agreeing with me. If two options are indeed essentially equal on all fronts, then one minor use case can be enough to push it towards a winner.
