As a project manager I need to be able to set the number of assignees per story for each story to correctly project out the tentative release date and team capacity.
For some larger issues it is realistic to have 2,3, or more developers working on a task but other tasks cannot be accomplished by developers working in parallel.
Additionally, some teams can work this way, some cannot. I'd prefer not to create a Portfolio Plan for each team only so I can configure different scheduling.
While it could be argued that a large task could be broken down into smaller tasks and set the "Maximum assignees per story" to 1 it's not always that easy when the requirements are not yet detailed to break down and/or you simply need to project across multiple future releases. This allows us to justify hiring early enough to onboard, and get up to speed in time.