David, you make some interesting observations. I think most of us would agree that controllers will only put an aircraft on a "standby" if they are busy. Prioritising your aircraft is something that is necessary - eg. I would offer that it is more important to assure a closing angle for an ILS than enter a discussion about providing someone a RIS/FIS. There are obviously occasions where this is not the case but a controller is forced to make a quick decision as to where his priorty should lie.
If an aircraft is on standby for a number of minutes, it is for one of two reasons: either the controller is really busy or, unusually, he has forgotton the initial call. The bottom line is that many ATC units no longer have the resources necessary to provide services to much more than their planned traffic (arrivals/departures). Consequently, it really is something of a lottery for people who wish to avail themselves of either a CTR transit or ATSOCAS. A sorry state of affairs but the harsh reality.