I think it depends on the creator, the story, the audience...
Burnout is a very real danger for any creator, and there's nothing wrong with giving oneself a break for some relief, to avoid that pitfall. That said, I do think it's best to keep crucial knowledge to the main series. But there's nothing wrong with adding to or enhancing things in side comics, whether or not it involves the same characters.
It's also probably worth noting that just because something may seem to be a side comic, that doesn't always mean it has nothing to do with the main story. Sometimes events happen that aren't tied in until much later. I'm not typically that much of a "long game" planner, but I've known people who were; it's worth giving the creator the benefit of the doubt.
I honestly just don't see why people can't let themselves just enjoy what's being done, really. If I'm reading something, I'm reading it because I like the voice of the creator and how they present their unique vision. Usually it only bothers me if the whole work is suffering in terms of enjoyability because of these side jaunts, which does happen, although in my experience only rarely.