This size estimate is extremely crude, but shouldn't underestimate the actual answer size by much. This would allow firewalled organizations with several SMTP relays to control the load distribution using the Weight field. MX 1 mailhost.ip-provider.net. 120 bytes for the 6 A RR's mentioned by the SRV and NS RR's. The authors believes this RR to not cause any new security problems.
Some problems become more visible, though.
It becomes impossible to block internal clients from accessing specific external services, slightly harder to block internal users from running unautorised services, and more important for the router operations and DNS operations personnel to cooperate. This could lead to denial of service. The authors do not see any practical effect of this. We assume that as the DNS-security people invent new features, DNS servers will return the relevant RRs in the Additional Data section when answering an SRV query.