When you let everyone reserve desks equally, you disrupt the hierarchy. It removes some of the empowerment that comes with leadership by tearing down walls and putting everyone on the same level. This may be a positive thing in some company cultures, but it could cause disturbances in others.
For example, someone who has worked at a business for 15 years and previously had a private office might be working adjacent to a new hire. While that might make the new hire feel good, it could be discouraging to the person with 15 years of experience who worked hard just to earn their office.
The disruption of the hierarchy can also leave employees confused about who to talk to with an issue, creating stressful situations.
Solution: Clear communication, team structuring, and flexible work arrangements for those who request them are good ways to address the issue.