(Someday, this may turn into a formal IESG Statement. For now, this reflects the current thinking of the IESG on this topic.)
According to RFC 2850, "The IAB acts as representative of the interests of the IETF and the Internet Society in technical liaison relationships with other organizations concerned with standards and other technical and organizational issues relevant to the world-wide Internet. [...] Individual members of the IETF are appointed as liaisons to other organizations by the IAB or IESG as appropriate."
Because IESG members and WG chairs are often the most well-informed members of the IETF (both procedurally and technically), it is tempting to appoint them as liaisons to external organizations. However, being a liaison can present serious problems for ADs and WG chairs: Because they both do technical review of IETF contributions and are the final judges of consensus in the IETF, an AD or WG chair who is the liaison to an external organization may be in the untenable position of advocating an IETF position to the external organization and then subsequently reviewing contributions made by that organization in the IETF and judging the consensus on those contributions. In this light, the IESG believes the following:
When possible, ADs and WG chairs should not be liaisons to external organizations. If someone acting as a liaison is subsequently appointed to the IESG or as a WG chair, it is often best that they step down as liaison.
In the case where it is important for an AD or WG chair to act as a liaison, there should always be a second liaison available who is able to step forward and take over that task should the AD or WG chair need to step back because of their IETF duties.
Often, it is important to have an AD or WG chair giving input into the liaison relationship. In these cases, the AD or WG chair may wish to not be the official liaison to the organization, but may actively consult with the liaison and (in the case of in-person liaison situations) attend meetings to feed important information to the liaison.
In all cases, whether the liaison is an AD, a WG chair, or another person, the IAB must be prepared to step in to represent the interests of the IETF should an issue need to be "bounced upstairs". In particular, ADs and WG chairs should not be required to be in the position of making high-level management statements in their role as an IETF leader: Certainly an AD or WG chair can make such statements if in their judgment it is appropriate, but an they should always have the option to remain "technically and procedurally neutral" and have the IAB take up the high-level liaison role. Of course, how the IAB takes up that role -- having an IAB member, or an ISOC staff member, or another IETF participant be the designated "high-level liaison" -- is entirely up to the IAB. However, the IAB must have the responsibility and must be prepared to supply such a person on short notice.