Kinda late to this, but actually there was no intention of keeping it hidden, or fully public in either respect. You are perfectly able and allowed to access and use the api.
For this, I suggest a few things:
1) Use this, its a pretty solid wrapper for the api.
2) If you really need to use the login portions (aka authorized) of the api, I think you're doing something wrong.
They did recently redo the api, to use slg's merchandise endpoints for addons, due to the market update, that information is included with the wrapper I linked above.