LotR2-TTT-Seq10

[Tuor of Gondolin]

Why should Saruman care whether or not the Heir of Isildur is with the Rohirrim? It obviously does not affect his battle preparation at all.

In a book sense perhaps little, but in PJ's movie it has
more importance, since Saruman is portrayed as
a willing lieutenant and willing information conduit
to Sauron (and not the duplicitous book Saruman)
and movie Sauron would hardly eye the return of
Isildur's Heir with favor.