I don't know why such a silly argument is so oft-repeated - Clinton's supporters are known for their lack of enthusiasm. Anybody (reasonable) who watched the primaries knows this, and this is likely to be even truer of the wider electorate, since they are faced with a choice between Trump and Clinton, as opposed to one between Clinton and Sanders (and therefore more likely to vote tactically). Fortunately for Clinton, it doesn't take much enthusiasm to put an X next to her name.