I would sympathise with the protest vote argument if the US had runoff elections, wherein there really were only two choices. If the presidential election had a runoff exclusively between Clinton and Trump, I would understand the need to support the former over the latter, since there would genuinely be no other choice, and any blank or spoiled votes would be actual protest votes. Fortunately the US has a different system, and if Americans are set to end up with either Clinton or Trump as president, they only have themselves to blame. They have a politically progressive (albeit inexperienced) alternative in Jill Stein. They have a very experienced and moderate alternative in Gary Johnson. They have an abomination of a candidate in Donald Trump and the epitome of DC corruption in Hillary Clinton. To vote for them, even when one agrees that they are bad candidates, is to consent to the perpetuation of this flawed system.
I am well aware of the arguments that Trump is too great a threat to be voting for third parties, and I have carefully considered the pros and cons of this particular election, but the truth is there comes a point when that kind of fear mongering just doesn't cut it. The very fact that Clinton and Trump are considered legitimate nominees should be reason enough to vote for either third party. If your only reasons to vote for them are the letter next to their names or the fact that they are not the other person, then you shouldn't be voting for them anyway. It's as simple as that. Wasting your vote in flawed candidates when there are other options is a good way to forfeit the value of your democracy.
Edit:
Obviously this is all assuming you're someone who doesn't like either major party candidate. If you genuinely like Clinton or Trump, or just like them enough to vote vote them this time around, by all means carry on. My above argument goes for someone who's only planning to vote for them because "third parties can't win".