I believe the cookie expiry times are set like that because the intention of those plug-ins is to remember the value for the user's lifetime, or 31536000 seconds.
E.g. the getVisitNum() is supposed to return a visit number for the user's lifetime, so it shouldn't be reset arbitrarily, e.g. every 3 days.
On the other hand, given the state of cookie handling by modern browsers (Safari!), this is probably a moot discussion.