In a post earlier this week Doug Cornelius (now over at Compliance Building) hit the extranet nail on the head by identifying the primary obstacles and benefits to extranet collaboration. His post sparked a flurry of activity with Luis Suarez of IBM making enticing comments about LotusLive (I have yet to check it out although I do note that now it is, in fact, live).
At ILTA 08 I heard of some firm-provided extranets through which clients obtained some access to up-to-date billing and fee information. Although it had to be done on a one-off basis, the clients who received that information valued it highly.
In addition to the instance of the large deal that requires very detailed item-level security access control, products liability litigation provides another extranet use case. There a whole set of cases revolves around a recurring group of experts, expert reports, plaintiff's attorneys, coordinating counsel, and local counsel. From the defense perspective, there is great value in aggregating information from many different people (such as local and coordinating counsel from law firms) who are repeatedly interacting with the same players and some of the same documents; you would think that could happen more easily on an extranet, though perhaps the logistical issues you note are a challenging barrier.
For their own marketing and KM needs, firms such as mine may already have collected information about the matters that clients also need and would like to have. If firms can aggregate and display (on an extranet) in a robust way rich sets of information about a lot of cases they handle for a major client, they are providing more value to the clients. That may initially be more of an information-gathering and reporting problem, but even this type of information could be made more useful through an interactive and collaborative online environment.