We have recently seen an increase in customers wanting to bring a large selection of PST files into their MailStore archive.  It's quite common for people to create local PST file archives of their own email and historically this was a common way to reduce the data within the mailbox on their mail server. The problem we see is that over time many users build up multiple local PST files and as these are scattered around on various client machines it gets very difficult and costly to keep them all backed up.  If you also consider in this scenario there is also no way for users to search each others archive it makes it very difficult to meet compliance requirements.