I'm aware of the mechanics of the spoiler effect -- it's one reason why I've come to despise the two party system. It makes it extraordinarily difficult for a 3rd party to supplant an existing party, and it makes it difficult to instigate change from within a party. The stability that a two party system provides also means that entrenched corrupt elements in the two parties are much harder to get rid of.
I despise being told not to vote for a 3rd party candidate because doing so is the only way that I can make my displeasure with both parties known, and if I don't feel well-represented by the major parties, then I still have an alternative choice. If enough registered Democrats or independents vote for a 3rd party, then the Democratic party will have to start taking those voters' concerns more seriously. Ditto for the Republicans of course. This is particularly important now, because of how Sanders was treated in the Democratic primary. When both major candidates are terrible, it's not good enough to vote for the less terrible candidate. It's disillusioning. By demanding that voters choose between only 2 extremely unpopular candidates, the end result is further disgruntlement and feelings of disenfranchisement among the voters, which could further depress voluntary voter participation. In other words, when a voter doesn't feel that any major candidate or party sufficiently represents their interests, it's highly tempting to just say fuck it and not bother voting. Even more so if there aren't any viable alternatives.
In my particular case, I looked at the Green Party's platform and decided that it corresponded closely enough to my own positions. Not completely, but close enough that I felt adequately represented, whereas with the other 3 candidates, I had various other reasons not to vote for any of them. Although I'm not happy with the state of the Democratic or Republican parties, my particular decision was a conscious choice to vote for a platform that I liked, rather than an explicit rejection of parties & candidates that I didn't like. Closely related ideas, but not exactly the same thing.