"If you vote for a third party or independent candidate, you're just wasting your vote," is one of the phrases so often repeated it's assumed true. Not only is it false, but assuming it's true weakens our democratic institutions.
Telling people that they waste their vote if they don't vote for one of the major party candidates is a dangerous mindset because it's exactly that assumption that contributes to the abysmal candidate choices that many Americans have been so upset about for many years. And it's why so many politicians are able to make promises they won't keep once they're in office.
The politicians will take advantage of you when they know you'll continue to vote for them no matter what because you think to do otherwise is a waste. Once you start thinking that way, you've become a captured voter.
"Do whatever you want when you're in office; I'll vote for you again simply because your opponent is worse," captured voters are effectively telling political candidates.
If you want your vote to matter to a party, you must be willing to abandon them.