This could also solve the problem Github has where anyone with an account an "approve" a PR, but if you aren't a maintainer for the project your approval doesn't mean anything as far as actually getting the PR merged, but can be a signal to the original author that it is probably good, and to the actual maintainer that the PR is worth considering.

But with this, a non-maintainer could review be allowed to give a +1 or -1, but not a -2 or +2, and it is more clear that a "+1" isn't sufficient for actually merging the PR.