That way, I can treat the series of merge commits to trunk as the simple, linear overview of history, but when I'm bug-hunting I get small, clear commits to search through with git bisect.

It also means I get more useful blame output, as instead of huge hunks of a file being all from one giant bomb commit, I can see the process and evolution that the whole change went through as the original author pieced it together. That can be really helpful when dealing with obscure bugs or understanding systems with little documentation, by helping you get back more of the original author's thought process and seeing how their mental model evolved as they built the feature.