I know you probably won't even care about the answer at this point, but its my first shot at answering something at SO, so here it goes.
I've been using SCSF for about 6 months so far, so I am not an expert by any means but a couple of the things I've been mainly using Module Services for:
When making a "business service" call (which I assume from your question means making a call to a data store of some sort or calling a web service etc.) you
can use a service to attach any additional data that might not be
directly related to the business logic but necessary to be persisted
with the call to the database. One example I can provide
is attaching the currently logged in user credentials to a call to
web service or to the database for audit trail logging. You call the
Service from your presenter which appends any additional info and
proceeds the call to the next layer.
Another use for Services is to abstract any implementation of
UIExtensions away from your modules. If you are using a Ribbon for
example, you can have a Service which gets injected into your
business modules with methods such as "AddRibbonButton()" or
"AddRibbonGroup()" which your modules can call to add the necessary
user interface elements when loading up without worrying about how
its done.
Hope that helps!