When it comes to IT in your company (or most critical rolls for that matter) you cannot have everything hinge on one person. That goes for an internal company person or an IT systems provider.
A single point of failure in any critical system should set off alarm bells – before the actual alarm bells occur. In the case of personnel that failure could be the individual being unavailable due to sickness, holidays, the proverbial untimely bus or sudden job change.
This risk opens the business to potential failures and should be mitigated wherever possible. When it comes to internal IT staff (either dedicated or the guy wearing the 16 other hats who knows how to use excel) you need to know what the knowledge and skills that are with only one person and what parts of the role need to be backed up by either another employee or contractor.
When it comes to your IT provider you need to know that your system knowledge is with a number of people not a one man show, and you need to know that when something happens you’ve more than one number to call.