I think Saruman's whole purpose here boils down to a need to salvage something from the wreckage, at the very least to get out of Orthanc freely.  If he can talk anyone into agreeing with any of his various ploys, he's well on the way to walking out of there.  His voice is the only weapon he has that is going to get him released--lightning bolts and gouts of flame will only tick off the visitors and bring return fire!

   Theoden is basically there as the victor, to confront the agressor who has lost the war and formally claim victory.  One might assume that Isengard will naturally pass back into control of Rohan at this point.  But it's also a personal thing, Theoden wanting to confront Saruman for the deaths and destruction he has caused.  He doesn't really need to achieve anything here, more of just putting a formal conclusion to the war.

   And yes, there is the usual purpose of giving the bad guy one last chance to surrender and repent, though I expect that Theoden is really hoping that Saruman turns the offer down.

Matthew/Ugluk
http://www.larp.com/orcs/