You're completely right about stageplays.
My reasoning for using images from multiple-ending stories is that
1. We're already talking about changes in Tardis:Images and perspective and it would be faster and more efficient to go over the potential changes all at once.
2. As far as I'm aware these are the only stories which are nonvalid for reasons unrelated to the four rules. I'm sure that there are those who would argue that an image of Tom Baker on a bicycle from the filming of Shada is just an image of Tom Baker on a bicycle from the filming of Shada, but I believe it's much less controversial to say that Omega appears in Search for the Doctor or that Skaro Degredations appear in Legacy. The only reason someone would say that don't appear would stem from the fact that those stories are invalid, but those stories are invalid for reasons unrelated to whether or not they're set in the DWU.