1) at this point Saruman comes voluntarily...I'm not sure he really wants to
deal much with Gandalf, except perhaps to use his voice to sow suspicion
against Gandalf by the others present. He does want another chance to
talk with Theoden, hoping he can still assert influence and control over him.
2) As others mentioned before, it may be necessary to have this final
confrontation to completely cure Theoden.
3) Saruman is looking for any opening to gain an advantage...first hoping to
bring Theoden back into his fold so to speak, and if not that than to try and
drive a wedge between Gandalf and the
Rohirrim.
