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We only use them on local machines for cached Outlook and anything else we don't support. As you can guess this makes us really popular when people corrupt their own personal store of PSTs but it's in the user manual which they've all signed so not much we can do.
The one are they are used which I really don't like is archiving. The current policy is that a job is archived from public folders to PST(s) and then the PSTs are dropped on the data archiving servers. Not the best policy but it's the best we can do at the moment ...