In an open source project, there are two important barriers that should never rise up: transparency, and entry.
Having gerrit be publically accessible ensures that there is a very low barrier to entry (i.e. that any new contributor can be referred to a contribution, new or old, and view it without signing up) thus contributing to good transparency of the project's actions.
I've linked a number of people to Gerrit reviews in the past few days, and a lot of them have immediately said things like: "requires login", "it's not public", ... etc. - so even if registration is easy, having Gerrit passworded IMHO does not make this immediately obvious, and should be changed.