Our attitude isn't "multiple contexts is not true GTD and therefore is verboten by the word of Davidco" or anything. We just think that the current approach, combined with the search field, covers the majority of our customers' workflows sufficiently well
for the time being.
There are absolutely some edge cases out there - and for some folks, those edge cases are the majority of their work, so I can see how the current situation could be irksome. For the customer base as a whole, though, we don't think this feature should cause us to ignore what is by far the most common feedback we've gotten over the last couple of years: "make the app simpler and easier to use".
In our opinion, the additional benefit for some people isn't worth the additional complication this would add for everyone. (Either the feature is enabled for everyone, or we give the folks that don't use it another preference to look at/worry about/accidentally enable/be confused by. We dislike both of those approaches.)
Our plan has been and continues to be to stick with a single-context approach, but make the database even more search-friendly. See
this post by Ken for a description of what we'd ultimately like to do.