Persistent values and class variables

I need to have a variable that points to a MemCache instance so I looked
at the Rails caching code to see how they do it. They use a Ruby class
variable (or a few of them). That surprised me since class variables in
Ruby as so gnarly.

I thought I would post the question here to see if anyone knew / had an
opinion. Why would they use a class variable instead of a class
instance variable? Especially since Rails has some nice support for
class instance variables.

I'm asking because if there is a reason, I'd like to know so I could
avoid whatever problems the Rails guys are trying to avoid.

Thank you,
Perry