Currently, the ability to use the Ranking field is based on the Scheduling or the Resolve Issues Permissions.
In some cases, an organization need the ability to separate these permissions.
For example:

• I have two group of users, let's say 'groupA' and 'groupB'
• 'groupA' needs to set up the 'Due Date' for tickets ONLY (unable to rank an issue). Then, they need to have the 'Schedule issues' permission.
• 'groupB' can Rank issues ONLY(unable to set a 'Due Date'). They also need to have the 'Schedule issues' permission.
• The problem is that when both of them have the 'Schedule issues' permission, they will be able to Rank the issues and also set the 'Due Date' at the same time.

Having an additional "Rank issue Permission" would cover the need.