I've seen this play out a couple time. I agree about a lot of what you said and this is indeed true that you can have very senior and very knowledgeable devs basically "hack" or "bulldoze" their way into a backlog, I would personally argue that this is not a decent or desirable behavior.

There is no such thing as "small finition". Making sure that a change or a feature works all the way through is not finition, it is core to the task, and you can't expect someone else to digest and do the latter half of the work without being in your head.

I guess I am too lazy to type out all the examples with the downfall, but basically if you allow this, you will be shielding a senior from his own butched work, and lets be honest, most people who do this skip the "boring" work for their own selfish reasons. If they want to split the task and have you fix the tests, have them spell it out and justify it.

Management might not understand what is going on, all they might see is a superstar flying through the backlog, while everyone else struggle because they're constantly fixing this guy's shit. This rarely lead to good engineering, or team dynamic, or team management, and of course you end up with this one guy claiming credit for so much shit, while other team members stagnate. Unfortunately appearance is a thing in dev work, as much as I wish it wasn't.