Would Saruman have been a reliable ally?

[Inziladun]

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.

Since Saruman knew so much of not only the Ring, but also of Sauron (through study of his works and mental communion via the Palantíri), I think he would have been of much help in devising military tactics for the West that would have been likely to divert his attention from Frodo and Sam.