Sure, but you can have plenty of options where there's one responsible person, but you may still want to track another person who's involved. Maybe another engineer who's paired on the project, or the PM who's taking point on it, or the QA who's testing it, or the other engineer who's reviewing it.
Having to rely on comments or assignment history to uncover that information is a bad experience.
Perhaps? But I do plenty of projects where I'm just paired with another engineer. I'm maybe taking lead, but they're sitting in on all the meetings and helping make decisions when I want someone to check my work. That may be worth noting in a ticket in case I'm busy or out of office and someone needs an answer around the project, even if they don't have a specific task they're responsible for that they'd be assigned a child ticket for.
It's also a level of precision question. At some point, having a lot of child tickets is just extra work if it's mostly just documenting something that could have been as easily documented by attaching another person's name to the parent ticket.
Having to rely on comments or assignment history to uncover that information is a bad experience.