Very few people speak at conferences as a hobby. Even fewer speak at conferences as hobby about things they get paid for. Even fewer would be legitimately willing to do this if it included an expensive middle-of-the-night flight, jet lag, and a Visa application process.
I've done it. Of course, I didn't travel internationally just for the conference and I didn't need a visa, but I've attended tech conferences on my own--including speaking. Why not? And how many Europeans who attend FOSDEM or Americans who attend Flock (admittedly a lower bar than traveling internationally) are doing so on their own?
ADDED: And people with hobbies/interests like, say, medieval history absolutely will attend a conference/event as part of a vacation even if it has zero connection to a day job.
Generally, when you apply for a visa, there's documentation guiding you towards the correct type of visa based on purpose of visit. Everything I've seen buckets conferences under "business visa".
I know governments aren't always commonsensical but it would seem odd if someone (not in the business) would need a business visa to attend Comic-Con or similar.
That changes the parameters of the hypothetical. Your original hypothetical was explicitly about misrepresenting the purpose of one's visit to a country, which one should not do because that's illegal. *OBVIOUSLY*.
Whether attending a conference for a hobby counts as a vacation trip or a business trip would be a question for the destination country's consular affairs office.