Hmm, I don't really understand this. How is this used on real teams? Doesn't having an unhappy face next to your name signal "I AM A PROBLEM EMPLOYEE"?
We use it in our small distributed team (Transparently) and it helps us figuring our when we have an issue worth discussing in depth.
You can anonymise it on larger teams and use it to figure out what actions may have made your team less happy, etc..
I'm part of the team that built Retroospect. Teams can be selected to be transparent or anonymous. Users who are invited on anonymous teams will not have their identity associated with the data - you will simply be team member 1. We see this as valuable for exactly the reasons you cited - to ensure that honest feedback is always given - something we feel doesn't always happen anonymity isn't catered for.