Full node may not be a requirement, but I'm definitely going to support the witnesses that elect to run one over one that doesn't - those full nodes are what keeps the lights on so to speak.
If the witness rewards aren't enough to fund the ongoing network infrastructure, then there's something wrong with the network itself, or the token value.
Hive is incredibly sustainable right now. I hope the witnesses are saving for a rainy day in case it comes.
It's definitely a plus and shows that the witness owner want to add value to his/her participation. Pure coincidence, @roelandp just launched his own full node cluster made of 3 hefty servers: https://hivean.com/hive-139531/@roelandp/announcing-my-hive-full-api-cluster-hive-roelandp-nl
The requirement I mentioned is what is defined in the whitepaper. But the community is voting for witnesses so the community can set the requirement.