I can appreciate the interest in probing further to determine whether or not our processes are setup around best practices. Suffice it to say: it is not, but rather a temporary work around as we work on a larger scale fix to a multi step process that's now in place.
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We just got a response from MKTO support that the Lead Score field is a "special field that Marketo will always override. " That was not always the case, so we're now digging to find out when that changed (since there will be some data cleanup required on our end.) In the mean time though, it does appear we'll have to use the Sales Insight option or devise a trigger around another custom field change.