Actually, its not a question of just webservice but lot of other features are yet to be released by SAP.
Even if some are released, partners are not enabled to use them...
Reasons for restriction can be anything...
They might be working on that and so they couldn't allow the public usage on that object.
OR
say, they might think that it may be too sensitive for a partner to handle coz partner actions can disturb any erp process....
OR
can be something else....
But, there is always a provision to request SAP with valid business use case so that they might consider releasing it on seeing its importance in market operations and demand.
Who knows? If your use case is so strong, then, your today's request may help lot of partners in their future projects. 