There's a difference between a "conversation" and an "internet exchange".
In person, I agree completely. Skepticism towards your friends/ family/ acquaintances can easily poison the discourse and ruin relationships. If I have a discussion with a person I know, then I should absolutely give them the benefit of the doubt, and take what they say at face value unless they say something I find literally incredulous.
When you're having a discussion online though, it's a completely different story. People say stupid stuff on the internet, and "stupid" is a spectrum that goes all the way to "well-intentioned and seemingly-logical but poorly-researched and false". Online, and on a site such as this which aims for a certain level of discourse, every claim should be backed up. There's always somebody who disagrees that is going to read your comment, and you should give them a reason to reconsider their initial position.
This is not an easy thing to do. I believe a lot of stuff without knowing immediately why. This forces me to better account for my ideas, which helps structure my thinking. I'm not a strong enough logician to make infallible arguments, but by trying to convince an eternal skeptic, I only make my own arguments better.
[EDIT: I gave you an upvote, since you were in the gray. Some others had downvoted you, but that was undeserved, since I think your disagreement was in good faith. Other people, please don't downvote stale's comment.]