I like to think that the principle differentiators come down to compatibility and support and end-of-life dates.
Appliance pros:
1) Guaranteed support of all internal components, until the EOL of the appliance. I.e. you will not be caught out by any new in-compatibility of any individual component.
2) No need to perform your own very detailed compatibility checks between OS drivers and HBAs and NICs inside the server (i.e. the appliance). i.e. an appliance abstracts and encapsulates the compatibility.
3) Pre-hardened and performance tuned O/S and driver set.
4) Simple CLIsh to manage networking and SAN components, and very easy to configure.
5) Backup admins no longer need to be 'server OS admins', and can concentrate on administering NetBackup and no longer have to worry about 'server OS administration'.
6) All hardware support in one place, with one vendor.
7) All server hardware monitoring is built in, and in one place - and OpsCenter understands appliances and can monitor the hardware too. Therefore no need to install and manage your own monitoring agent, instead just point/send the appliance SNMP alerts at your chosen alerting system (HP OpenView, BMC Patrol, etc...). And you have MIBs for the appliances, whereas you do not have a MIB for your own server.
8) Symantec Support understand and support the 'whole package', reducing the number of bun fights with other hardware vendors re interoperability - i.e. less need to engage other vendors and get in to complicated three way support calls with multiple vendors.
9) Appliances have a WAN scaling feature, which roll-your-own NetBackup servers do not have.
10) Appliances MSDP pools can be 'larger', i.e. appliances can be grown much more than your own MSDP pools - therefore less 'servers' required as your estate grows.
11) The underlying 'storage management' i.e. placement of the of the various file-systems is much easier via the CLIsh, i.e. you do not need to be a VxVM/VxFS expert to move and re-size the internal 'partitions/file-systems'.
12) The appliances now have a built in mechanism to protect the MSDP meta-data 'de-dupe catalog' (not to be cofused with the NetBackup Catalog) which enhances self-protection, which I believe that roll-your-MSDP-servers do not have.
