Based on following this issue for over a decade, personal experience and discussions with the folks in the industry three reasons seem to rise to the top:
Originally Posted by andre marcoux
1. For loaded touring or any other heavy-duty requirements where constant braking would be required on a long, steep or long-steep descent, Co-Motion's recommended set-up has been proven over time to be the most effective. The reason for this is straight forward: the Arai drum is an application specific supplemental 'drag-brake' designed to deal with very high heat loads and that works in parallel on a non-interference basis with rim brakes. Note: At present, no one has designed a disc rotor with an integral drum brake so the use of one precludes the use of the other.
2. When Co-Motion first began to offer the Avid discs as an option Avid was still a small Colorado-based company who had a good relationship with the folks over at daVinci Designs / Tandems. The folks at daVinci did a lot of beta testing with Avid's 203mm mechanical disc brakes at a time when there weren't any road tandem-rated / approved disc brakes on the market except for a klugged-together and problematic mechanically-actuated, hydraulic Formula rear disc by Santana. Based on real-world results by the folks at daVinci -- to include doing the epic Mt. Ventoux and Alps du hez on an Avid BB7 equipped daVinci tandem -- Avid agreed to characterize their discs as suitable for use on tandems as a primary brake so long as only the 203mm model was used. Looked at another way, while the Avid lacked the overall heat capacity of an Arai drum brake, it was still as good as or better than rim brakes. This allowed Cannondale to sell their RT models with dual discs and Co-Motion also offered up the Avid's as rear and/or front & rear primary brakes in place of rim brakes which are probably more than adequate for 90% of all tandem teams. However, for anyone who truly required a drag brake, see #1 above. If fact, early on if someone wanted rim brake mounts and a disc brake mount Co-Motion made a point of providing only one set of brake cable stops too preclude the use of the disc as a supplemental drag brake. Obviously, any resourceful consumer could easily modify their Co-Motion to accommodate a second brake cable installation to thwart Co-Motion's efforts to abide by Avid's guidance, but at that point it was on the owner not Co-Motion if they go into trouble.
3. With few exceptions, if a couple buying a tandem is candid about how they'll use their tandem any one of the standard brake packages will usually be more than adequate to meet their needs. True tourists know who they are and will eschew the trendy discs or racing brakes and gladly use the rim + drums on their tandems along with mud guards and racks and the same usually holds true for folks who ride multi-seat tandems (triples, quads, etc). Racers and go-fast riders will use the same brake package they have on their single bikes, e.g., dual rim brakes and calipers where possible. For racers and faster recreational riders who venture into mountains with technical descents, a rear disc offers a belt and suspenders approach that gives them added brake-heat capacity to deal with the unexpected (yikes, I didn't realize how gnarly this was) or that special descent on a certain ride they do where something better than a rim brake is called for, but a drum brake would be overkill. All of the other 'options' are simply variations that may or may not be any better than the 3 basic configurations. In fact, there are even folks who have rear discs as a primary brake running around with a stoker-controlled rim brake.
So, as a consumer you're free to buy from the slate of options that Co-Motion and anyone else offers. If you've been successful using a front caliper and rear disc on what you consider to be loaded touring and on terrain that represents the upper end of what you will seek out in the future, then you could probably assume that a similar configuration on a Co-Motion would work just as well (WinZip 10" isn't all that more robust than Avid 8", despite the marketing spin) or even dual discs. However, that's a personal choice that you're free to make. Co-Motion, on the other hand, must apply a level of conservatism as an OEM and that's what forms the basis for their recommendations.