A recent, small, but useful fix for simplenote.vim to help alleviate this issue. I.e. this is one of those issues with Open Source software and using API access tokens. Oysttyer sees the exact same problem (I just have no time to work on that any more). There is just no way not to make the token visible which leads to abuse (fucking human beings) and being rate limited or blocked.
I honestly did not realise though that Simplenote/Simperium authorisation tokens were basically permanent so it makes sense to cache and re-use. That way if the API token runs into issues again it doesn’t really matter.