About the fact that these fields are more for other bundle items: you are right, those settings are quite always there, but I think they have to have a functionality. Otherwise why not removing them? (e.g. Language Grammars has different/more fields. (Although those one are always there…)).
We prefer to keep things general because they can be used, and some users do find creative ways to do things.
In the case of the scope selector, this limits the scope in which the key equivalent or tab trigger has an effect.
So for example in the case of themes, let’s imagine we have two themes, theme A and theme B. We could assign both of these themes a key equivalent of ⌘1 but different scope selectors, e.g. source.c and text.html.
What this would allow is to press ⌘1 in a source.c file and switch to theme A, yet in text.html the same key would switch to theme B.
If someone would actually find this useful, I do not know, but knowing that all bundle items can (more or less) be treated the same, I think has value in itself.