This is the reason that they all ship with those obnoxious doser guards, hopper screws, hopper guards, etc. I think that speaks more to the chute geometry than to the cavities that Bryan is referring to.
Im not convinced thats a real substantial amount of coffee for FIVE commercial grinders. In a commercial enviroment like they were designed to be in, accumalation is nearly a non-issue with a good barista like you who actually cares about cleaning it out on a regular basis. Look at it this way, at least the Mazzer is designed well enough that it is easy to dis-assemble and clean easily and at least it's not a large conical(think robur/k10) which holds MUCH more gounds in the chamber.
I just got a new K30 for my home lol, so maybe its not fair for me to comment lol;) anyways, props for keeping your grinder clean. I know what you mean about douching out a Mazzer after heavy use. Take pride in it brotha!
I have thought of the same question. I've taken the grind platter/burr assembly parts apart literally hundreds of times on many types of grinders and the situation is always the same. There is always this oily paste under the spinning burr and this hard clay-like substance under the platter. Both are quite difficult to remove.
As for the reasons I have to think it's along Brady's line of thinking. Manufacturing tolerances. And possibly heat dissepation. In order to make metal surfaces mate flat against each other it takes special machining. This kind of machining is time consuming and expensive. Most of the spinning platters start their life as a casting and are then finished on a mill. If the piece can be finished in one operation it keeps the price down. The only exception to this I've seen are the big Mahlkonig and Ditting grinders. Espessially the Mahlkonigs. When everything is put together the tolerances are exceptionally tight and there is very little room for the coffee build up. The burrs sit flat against the platter and when you turn the unit on with the top off it looks like it's not even moving. It's actually kind of cool.
As for heat disipation you've got two ways to go. More mass or less. With the Majors and SJ's (or with any other brand for that matter) the burrs are pretty small and the total amount of mass availible for heat transfer and dissipation is limited. Since the burrs are where the heat is generated if they have less mass they will heat up faster but also cool down faster. If the burrs mated flat to the burr carrier in an espresso grinder the whole thing would hold heat longer. Heat distorts metal and brings out the oils in coffee. Also the metals used in espresso grinders tend to be dissimilar. A brass spinning platter with steel (or titanium or ceramic) burrs. These materials all behave differently with heat. If you decrease the surface area between the dissimilar materials you will allow them to act more independantly. When the brass starts to heat up and distort it will not transfer that same distortion to the harder and more brittle steel burr. I think that is why there is space between the two. The same would go for the space between the spinning platter and the floor of the grind chamber.
With the big grinders like a Ditting or Mahlkonig the opposite approch is used. The burrs are massive and are flat mated to a massive spinning platter which is attached to the massive spinning motor rotor via a massive flush fit cone shaped spindle. I think the idea is that when heat is generated by the grinding it is transfered into this giant mass for disipation. Most of the time the heat generated is minimal in comparison to the mass availible and everything is fine. But if you are trying to grind a couple of five pound bags through a dirty grind chamber the heat build up is more than the mass of the grinder can absorb and then the thermal breaker pops.
The other thing I've noticed is that the difference between the compostion of the newly ground coffee and the wierd paste/clay that builds up under the burr and the platter keeps the two from interacting vey much. There is no doubt that if you made a shot with the crap you collected from the undersides of those burrs and platters that it would make you hurl. I just don't think you're going to get much of that crap in your grind durring normal use. It might be that your grinder is not broken in properly until those area are filled in. Who knows?