I try to respond to everything seriously for a few reasons:
1. Poe's Law - For those of you who don't know, "Poe's law, named after its author Nathan Poe, is a literary adage which stipulates that without a clear indicator of an author's intended sarcasm it becomes impossible to tell the difference between an expression of sincere extremism and a parody of extremism."
In the current age of internet stupidity, it's entirely plausible that no matter how ridiculous someone sounds, they may actually be that stupid - because previous experience on Tumblr has proven to us that there are people out there who really are.
So even though it's likely to be a troll, I'll take it seriously, just in case.
2. Extremism and Hypocrisy do not invalidate a point - They might make the speaker look the fool, but even if you an heroed, your original argument still stands, and still needs to be addressed.
In the instance of last night, however daft it is to call Hasbro nazis because a writer named a single-use character after a famous fashion designer who was in a relationship with a person who was on the Nazi side of World War II, it still begs the question "should they have chosen someone else to name the pony after?" or "is it unethical to appreciate Coco Pommel, knowing that she is named after a potential nazi sympathiser?"
So for the sake of intellectual exercise, I answer the question anyway - it's still a talking point even if the reason for asking the question was stupid.
In cases like this, despite point 3 (read below), extreme or hypocritical arguments sometimes just make the point even easier to resolve.
3. Resolve the extreme case and you have resolved all cases - When addressing any issue or conducting any test, it's always a good idea to test extreme cases to see just how far your theory can go without breaking.
If you build a bridge than can take the weight of ten cars, someone will inevitably ask, "What if eleven cars attempt to cross at the same time?"
If you can say "I tested it for ten tanks," despite the fact that ten tanks will never attempt to cross the bridge at the same time, you have instantly laid to rest all questions - asked or unasked - about the bridge's strength.
So I do my best to answer troll questions, because if we can resolve an issue taken to an unrealistic extreme, we have resolved all other questions of that issue.
It also discourages people attempting to use Slippery Slope / Appeal to Extremes fallacies, because they will see that I don't back down to hyperbole.
