Here is a use case where having multiple task owners would be a huge time saver without compromising accountability.
We are a web development company and our tasks are nearly always executed by micro teams consisting of a designer, front-end developer and back-end developer.
They have firmly defined roles and responsibilities - there is no confusion around who needs to do which part of the task.
What the system is forcing us to do today is constantly set up same set of subtasks over and over again, i.e:
Launch a new homepage
- Front-end development
- Back-end development
But this is entirely unnecessary for our team - the responsibility and accountability is firmly defined by individual roles, not by the system.
We would simply like to assign the task to all three of them and have the project manager check-in with the team and see how things are going.
Another problem with subtasks is the lack of context - you often need to navigate to the parent task or the project to get the details you need to complete the subtask.