Ask HN: Ever use someone else's Auth Token to access an API?

There is a website that has an API that I want to build upon. Their API is request only access, I requested but haven't heard back yet. However, their front end accesses their API, including sending requests with an auth header. I did a quick POC and was able to access their API by sending their auth header with them.

Has anyone done this before? What were the long term results?