First of all, I want to say that I would absolutely love to manage my other accounts from within Monzo and have one place to go. However, I’d rather wait for the PSD2 APIs before using something like this as I’m not keen on sharing my login details. It’s not that I don’t trust Monzo, but as @nanos says, I think it would put me in a bad situation with my other bank should there be an issue. It’s the same reason I haven’t used any of the aggregation services so far, not even Yolt which is backed by a major bank (ING).
But, please, I am really hoping for account aggregation within Monzo once PSD2 is up and running. And I would want write as well as read access then.