Is ADFS mandatory for Lync 2013 Hybrid Deployment?

We alreadys have Lync 2013 Onpremise.

We now wish to do a Lync Hybrid setup with O365 Lync Online but wish to know the following.

1. While doing a Lync Hybrid setup, is deploying ADFS mandatory? We ask this because the new DirSync tool has the ability to Sync users password and thus we can avoid deploying ADFS/ADFs proxy/ADFS farm etc, thus reducing Onpremise complextities. We dont care about SSO as far as users can Logon Onpremise or Online using their same AD password.
2. If we can do a Lync2013 hybrid setup without ADFS, then can we later have our Lync 2013 Hybrid setup to federate with our partners domain?