I understand the 'rationale'; philosophically I would agree, but in all practicality it's causing problems.
Consider the situation where several people can complete the same task and have equal co-ownership of it. Perhaps one person has "more ownership" of the task, philosophically, but in reality more people than one are accountable for things being done. And more importantly, people need to get an overview of all things that involve them. It doesn't make sense to have copies of the task, if its completion is global for all involved people. Nor does it make sense for just one person to "own" the task if it's not truly solely owned by that person. It forces the person to run around like Mother Hen to make sure it's done (costing time, attention and effort), even though that person at one certain hour, session during the day, or even that particular workday, may not be the person to tend to it.
Currently, we simply don't set any assignee to those tasks, because we don't want to litter our workspace with duplicate tasks, and it's not really fair to say that one person owns that task either.
Asana has a lot of bloat in its sidebar design, especially when you have a few teams with many tasks, and that makes it important to have a cohesive task Inbox/Personal page that can give overview of all tasks that can involve "person A", even if that task is shared with other people.
The design decision to force only one assignee is too heavily railroaded to make practical sense for all real-world use cases, and the alternative to "copy tasks" is really not an alternative at all, but should just be considered for what it is at face value – copy a task, nothing more.