The benefits of using boundary nodes would be that only boundary nodes have IPv4 addresses. We do not have sufficiently many IPv4 addresses to equip all IC replicas with IPv4 addresses. Thus, using boundary nodes is one avenue towards enabling IPv4 connectivity for the HTTP feature.
As you say, in terms of rate limiting, going via boundary nodes means that if a boundary node is blocked, it is not available for any replica to use as a proxy. But, as the number of boundary nodes would grow with the number of replicas, the problem would not be as pronounced as it seems.