1) If the PST is stored in the Local Computers & the Hard Disk fails, chances are that PST will be lost along with data.
2) If the Admin makes a global search, the mails in the PST will not be searched. So Compliance & e-discovery suffers.
3) The data in the PST is available only from the Outlook of the User. If the User were to login on a different PC, then this data is not available.