Would Saruman have been a reliable ally?

[William Cloud Hicklin]

Originally Posted by Tuor in Gondolin

It is interesting to speculate what kind of "great sevice" Saruman
could have rendered. Perhaps helping to protect eastern Rohan so as
to release more forces for the ride to Minas Tirith, say 8,000 vs.
the 6,000 brought.

Theoden left no able-bodied Riders behind; he knowingly and willing left Edoras open to attack and destruction, which would in fact have happened but for the Ents' unforeseen appearance and their destruction of the Orc-army in the Wold.