Be extremely careful with providing your active key to any other person or third party website. Anyone with access to this key can basically do whatever they want with your account, including stealing all of your money. You could still recover the account with your owner key, but any money they were able to transfer out would be gone. You should really trust anyone or any website that you are providing this key to. You are essentially giving them full access to your account. If you wouldn't trust them will full access to your account, then don't give them your active key.

It's not just trust in the service to not steal; it must also be trust in their ability to fend off a server or credential compromise.

I could trust the busy guys 100%, but if they get hacked through no fault of their own, any keys I provide to their site are just as stolen (by whomever hacked their server, in this hypothetical scenario).

You can't be too cautious here.