Ahhh ... the pain and pleasure of using MusicBee. It's so damn flexible and configurable that everyone uses it differently. And that's not a bad thing.
I've never experienced the "album split between the Inbox and the Library" so I never considered that as a reason for including the Inbox in searches. My Inbox use is merely a local stop on the express train to my Library. (Geez that's corny.) It's not full of lots of music. I usually know what's in it and can easily find something simply by scrolling through it. My Inbox is used for tagging and cleaning up albums/tracks and then moving them into the Library.
As for searching "Local Node Only" I've got 90+ playlists and eight filtered libraries. I'd have to know which node to search when looking for something. If I know which node to search, I probably don't need to search.
As with most new features, where some want and others don't, the best solution for something like this is to add an option to searching "Entire Library" to include or exclude the Inbox. Somewhat like this, but with the option being under "Entire Library" and with a tickbox and the wording "Include Inbox" or "Exclude Inbox".
I know Steven isn't too keen on adding option after option after option, but I think this may be a case where it would not add clutter.