while I would think that is the reason of existence of the whole application
Well, yes. But it also depends on how the filter is used and what for. AMOS is used not only by lang pack maintainers, but by community contributors too. So I was trying to make the UI a bit friendlier for them (as AMOS has been really designed just for the maintainers initially).
The available stats show that the field "Show missing and outdated strings" is checked in roughly 28% of cases. So users leave it unchecked in most cases and it makes sense to actually have it as an advanced option.
My other reasoning was that, for
mortal contributing users, it is actually more intuitive to see all strings in the selected component (translated and missing). The interface should work well even without "Show missing and outdated strings only". For example, pressing the tabulator jumps to the first red (missing) translation field. This allows the user to see the context of the translation.
And finally, with the field checked, you can easily get "No strings found" in cases when everything is translated and up to date. However, some users may be actually confused by that. So again I thought it would be better to keep this option invisible unless you really know how it's working and what it is good for.
I'm open to further discussion though - the last thing I want to see is annoyed AMOS power users