I have these discussion virtually everyday when talking to customers about Exchange, the Realities and also a new whitepaper discussing Large Mailbox Vision can be found here and here
Myth #1: Exchange requires expensive, high-performing storage . I can't afford large mailboxes!
Myth #2: Exchange 2010 doesn't support storage area networks (SANs).
Myth #3: I already have a SAN (or I just bought one), so it doesn't make sense to implement DAS. By the way, my SAN can use those less expensive SATA disks too.
Myth #4: JBOD configurations are not practical because the re-seed process after a disk failure takes too long, and this generates too much operational overhead.
Myth #5: Large mailboxes perform badly with Outlook.
Myth #6: When I migrate to Exchange 2010 my database size will explode because Exchange 2010 doesn't have single instance storage (SIS).
Myth #7: My Exchange guy knows nothing about storage - it needs to be managed by the storage experts. Less expensive storage is too hard/time-consuming/expensive to manage.
Myth #8: I can't backup large Exchange databases.
Myth #9: We need a 3rd party archiving solution because Exchange data needs expensive storage and we need to put archived data on less expensive storage.
Myth #10: All Exchange storage designs must follow the Exchange Mailbox Role Requirements Calculator verbatim, otherwise they will not be supported.