If your goal is more than just to convince them to vote trump but to radicalize and spread ideas r/the_Donald is the perfect place to do it. If you look at how a lot of conspiracy theories spread from there I can see it being very enticing

Throughout the election they gamed reddit to get their stories to the front page. It went far beyond /r/the_donald.

It's a good laboratory to test out memes and narrative framings for distribution more broadly. It basically winds up working like a big genetic algorithm for developing viral content.

Also, while the extremists might not be able to vote in numbers to turn an election, they can definitely make the internet a sufficiently unpleasant place for discussion that they have a chilling effect on support for everyone else.

Many Clinton enthusiasts were hounded into "secret" or private subreddits and Facebook groups because of the intensity of abuse they received. Most of the more leftist and Bernie Sanders supporting spaces were, likewise, spammed with so much FUD content that it was impossible to have a productive conversation about anything else. They were a seriously disruptive influence on the capacity for anyone else to organize.