@RatherBeGolfing Just to be clear... optional aggregation is permitted in 3 circumstances stated generally as: (i) employers using a common paymaster (not that many employers actually meet the common paymaster rules because it should only cover "concurrent" employees), (ii) an employer and one or more other employers in a 414(b), (c), (m), or (o) control group, and (iii) for successor employers in asset purchase. See § 1.414(v)-2(b)(4)(ii), (iii), (iv) Federal Register :: Catch-Up Contributions
Administratively it could be an issue where several controlled group members participate in the same plan and employees work across multiple entities in the controlled group or are transferred between the entities. Without aggregation, mid-year transfers could cause the employee's catch-ups to be characterized differently, so there will need to be more communications with the employees to minimize confusion.
Certainly more reasons not to do this but it depends on the employer and their structure.