Mewiet wrote: Time Crash has to list the Fifth Doctor as "featuring" even though both Ten and Five play equal roles in the story. The Day of the Doctor also features Eleven, Ten, and War with equal roles in the story. Four Doctors features Twelve, Eleven, and Ten in equal roles. I'm sure there are more, but those are off the top of my head. Really the only difference is who happens to be the incumbent actor/Doctor at the time the story is released. And that doesn't even work for things like Big Finish releasing multi-Doctor stories where the incumbent Doctor doesn't feature, because then who should get to be listed as the "main" Doctor?
Tennant is the main Doctor in "Time Crash" because he is the current Doctor at that point whose adventures the audience is following through it from "Last Of The Time Lords" into "Voyage Of The Damned." Davison shows up in Tennant's console room because Tennant was rebuilding the TARDIS and forgot to put the shields back up, and Tennant provides the solution because he remembers seeing himself do it through Davison's eyes. The loop begins and ends with his actions.
Likewise, Smith is the main Doctor in "Day," and the others are guests. Again, the audience is following him at that point and the story begins and ends with his perspective on things. It is indeed a function of him being the incumbent, sure, but that's the way the story is structured.
However, I do agree that this doesn't necessarily apply to Big Finish stories such as The Sirens Of Time, for example.