Would Saruman have been a reliable ally?

[Tuor in Gondolin]

Originally posted by William Cloud Hicklin
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.

Theoden did leave some forces in Rohan, partly because of the necessary speed to get to Minas Tirith, but also to keep some forces protecting Rohan.

'But we will speak no longer counsels of prudence. We will come. The weapontake wa set for the morrow. When all is ordered we will set out. Ten thousand spears I might have sent riding overthe plains to the dismay of your foes. It will be
less now, I fear; for I will not leave my strongholds all unguarded. Yet six thousands at the least shall ride behind me.'

[Mithalwen]

Tuor, I think WCH means when they rode to Helm's deep not to Minas Tirith