Hi.
I second this: rules have to be enforced by technical means whenever possible. Social conventions simply don't scale: if we let people just merge it arbitrary than there'll be never-ending stream of "I know I shouldn't have but..." with all sorts of "buts" (I was in a hurry, I was about to leave, I needed for X, etc).
It might work fine in a close-knit group but I think the goal is to grow the number of contributions by making it as effortlessly as possible. It's always better than newcomers simply can't do the wrong thing rather than constantly reminding them what is right thing, on which wiki (which inevitably will get outdated) they can read about it etc.
Also, I disagree that setting up gerrit this way is a "hack": making votes additive is described in official documentation for gerrit. It's supported by upstream - there's nothing hackish about it.