You utilize collaboration services like email to stay connected. It is not an optional component to your operations. How much time do you and your staff expend taking care of spam/junk email? How often has a workstation or other operation been compromised by malicious content? Have anyone you known, including yourself, been victimized by some entity using your account to send virus emails to all of your contacts?
What happens to your reputation when you have been compromised?
Are you in a regulated industry and need to archive your email? Are you concerned about E-Discovery and harassment suits? Are you archiving spam or real email? Do you need to send confidential information via email?
Does your mail service ever go down? How long does it take your staff or service provider to perform maintenance or upgrades? Has a weather storm ever affected your operations? Would having a service that automatically took over when you systems were offline be valuable?