My own angle on chambering movements is based on a general principle (which applies to much more than just the MAs) that you start from the strongest premise and see how far you can go in defending it, rather than a weaker premise. In the case of form interpretation, the strongest premise is that all movements have combat application (typically, several different ones, depending on how the preceding and following moves are interpreted). The reason this is the strongest premise is that it's compatible with a much narrower range of facts than the premise that not all combat moves are combat-effective (i.e., some are decorative, or are for training, or are expressive of some spiritual/philosophical/cosmological principle, or...). The stronger position is the one that's most easily shown to be mistaken, i.e. It's the same idea as when you see two critics arguing about the interpretation of a certain passage in an Elizabethan play: one says, well, it's hard to make sense of this bit, so let's just assume it's a textual error; the other says, no, it's just what the playwright intended, but you have to understand the play in a different way, and then the question is, is there independent evidence that supports that different understanding? In the case of chambering, my assumption is that the chamber is often the 'business end' of the technique revealed in the form, so then you have to decide just what that technique is.
We already know that the labels that are attached to KMA and JMA striking arts aren't to be taken literally: Itosu in his own writings was explicit about that, and made it clear that describing things as blocks and punches—a description tradition which he appears to have started, in order to make karate acceptable to the overeers of the Okinawan school system—was intended for teaching karate to children; that the actual techs behind the form of the kata were much nastier than those simple kihon moves. So when we look at a movement, it's an open question what the intended move is. Where the teaching transmission has lost the original intention, the intention that shaped the form of the combat sequence in which the movment appears, we have to try to recover it from considerations of practicality (with respect to our understanding of how actual fights develop) and effectiveness (does it work to minimize the length of time the defender is in danger?). Stances, chambering movements, so-called blocks, so-called punches... all are subject to the same kind of scrutiny.
With chambering movements, there are a number of very natural interpretations in which the chamber is a crucial component of the technique. In the 'down block', for example, the retraction chambering, taken to be a wrist grip/twist pulling the attacker's grabbing arm toward the defender's hip, in conjunction with a rotation of the defender's body, helps set up a second trapping movement where the defender's forearm, slammed into the attacker's extended arm just above the elbow, creates a pin which forces the attacker's head lower the more weight you project into the pin (by a so-called front stance movement). The defender's pinning arm, if it's released quickly and raised high to the defender's opposite side, looks as though it's 'chambering', but the hard, fast 'down block' that creates a hammerfist strike to the attacker's temple, carotid sinus, collarbone or etc. isn't exactly a simple 'preparation' move, expecially since, it can be broken down further into a sequence rising-elbow-strike (to one side of attacker's head)—downward-spearing-elbow-strike (to attacker's face)—downward-hammer-fist (into attacker's temple, throat, etc.) So what look like a retraction chamber of one fist and a set-up chamber for a simple down block winds up having plausible, effective interpretations (in, e.g., Palgwe Sam Jang) as pinning/trapping movements and multiple strikes to vulnerable points on the attacker's lowered upper body. In the Abernethy discussion I cited earlier, the use of the rear hand chamber as a crucial component of a deflection, letting the defende go inside and deliver the forward knifehand 'block' to the attacker's throat again illustrates the way in which the chambering part of a movement can in fact be seen realistically as a component of a decisive combat application.
My feeling is, if some movement , whether a chamber or anything else, doesn't have some effective application, it's going to show: the interpretation of the sequences in which that movement, interpreted as a particular move, is embedded will grow more and more baroque and unrealistic, and at one point it'll be clear that the point of the movement was something else than a specific element in fighting tactics. But it seems to me to be a good idea in general to start from the assumption that there's combat utility in a chambering movement, and see how that holds up in thinking about each subsequence of a form. After a certain amount of experimentation, you may well decide that the intention behind the move was not directly relevant to operations in a fight, and if that's the case, so be it.
