I'm not in the industry so please forgive me if I'm being dumb or ignorant, but there does seem some sense in requiring insurance docs to be carried. If [Obscure]Air turns up from [Obscure]Land, you've got something tangible that you can then check up on if you wish. Without it you've only got a promise from [Obscure]Air and a hope that [Obscure]CAA is strong on regulation.
As I say, just an outsider's observation/question.