The mailbox server running OABGen also has another role, to generate group metrics. Group metrics counts the members of all groups periodically on mailbox servers, and uses Exchange File Distribution to get the group metrics files out to CAS servers. Exchange File Distribution is the same method used in OAB distribution.
The Exchange 2010 mailbox server will have a share called \\mailboxserver\GroupMetrics where your group metrics are generated. If you're not seeing that on any mailbox servers, you're not generating group metrics. When you look in that share, make sure the files in it have been changed within the past 24 hours. Restarting the Exchange Service Host will generate a fresh file; that process may take minutes or hours depending on the number of groups in your organization. Also, make sure the Exchange File Distribution service is running on Client Access servers and Mailbox servers, and that your CASes can connect to \\mailboxserver\GroupMetrics.
It is possible to separate the mailbox servers generating OABGen and Group Metrics. To configure a different server to generate Group Metrics use the following command:
I do not recommend this however as both the OABGen and Group Metrics generation roles cannot be clustered or made highly available in anyway. As a result it is best to put both these tasks on the same box so your mentally aware of the single point of failure (SPOF).
What are Group Metrics used for?
Group metrics counts the members of all groups periodically on mailbox servers. Group metrics is used by a Mail Tip which counts the number of users your about to send an email to.