I think it makes a lot of sense for SysAdmin type work; there are times when you really need me /right now/ - waiting for me to clear my schedule some time next week is not going to work.

I think maximum value would be achieved if I put your stuff on my pager. when something breaks, I go fix it and let you know what happened when you wake up; This is a lot of value for you, but for me to be effective at this, I need to know a lot about your systems, and as such, I'd want some sort of monthly money to pay to hold your system in my brain and my notes (and to answer my pager when it goes off at 6am.)

It seems to me like this sort of relationship would work best if it was very long term; you'd keep paying me the retainer until you can afford to hire your own SysAdmin.