The answer is pretty technical. The simple version is that if the player can't detect film mode properly, it also doesn't have the information necessary to stitch together the correct fields. If you force film mode, you get awful combing, because it goes ahead and starts weaving fields together blindly. For this reason, it's generally not a good idea to use the forced film mode; it's unlikely to work any better than Auto.
There is only one player we've seen in which film mode is useful: the JVC XV-D723GD, because it doesn't really have any film mode detection, so for a few films where the frame structure is correct, but the flags are not, putting the player into film mode fixes things for the majority of the film. Specifically, I'm talking about Titanic, Austin Powers, T2 (the first release) and other films with the alternating progressive_frame flag problem. There are many films with this problem, but those are the highest-profile ones.
And even on those films, they drop flags or cadence from time to time, and then in film mode you get combing. For example, on Titanic, the progressive_frame flag drops entirely for about 20 fields right around most of the chapter stops, and the 3-2 cadence breaks as well.
Stacey and I are both of the opinion that at least half (and probably more) of all the DVDs out there have flag and/or cadence glitches in the main feature. They all have difficulties in the supplements, but I think the average viewer cares more about the feature. That said, probably for most of these films there are at most a handful of glitches, and depending on the player, it may just sail right through the glitch, or comb for a single frame, or comb for multiple frames, or drop to video mode, and different people are sensitive to different things.
All of this is covered in the report, and hopefully it will help people avoid the problems they find most distracting.
[This message has been edited by dmunsil (edited 12-20-2000).]