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You can enable quotas without enforcing them and see who (again, based on file ownership) is using up all the disk space. NOTE: sometimes admins "take ownership" to gain necessary access to files - if they do this, it skews the quota values since now the admins "own" the data. When you have limited resources, quotas make sense.
For example, if you have 3 classes of data - CRITICAL, Important, unimportant (relatively speaking), then maybe you don't backup the unimportant often and you use cheap SATA drives without RAID. This allows you to provide lots of storage at low costs so putting a quota on it may not make sense. The Important data may be on a set of server class SATA drives with RAID... this "drive" may have quotas enabled but not enforced so you can get a quick report of who is using the space and "yell at them" if they use too much or others need space and there's not enough because a few people are using too much. Finally, the critical data could be on FAST, EXPENSIVE SAS drives in a RAID 10 and so you might want to limit the initial amounts of data all users can store there so you ensure everyone has enough space to store their critical data.
Making a case depends on what your business does, how abusive of the resources your employees are, what kind of authority the IT department has to dictate to the users (IT should SUPPORT the user needs - but this is best done on a management level - managers in other areas should go to IT and have systems put in place to better support their staff within budgetary constraints. In a research environment, researches can sometimes DICTATE to the IT department what they need and as long as they can provide the money, the IT department may HAVE to do as requested.