I agree with Rayne. I reckon if Saruman hadn't been utterly destroyed (well, not him, but his forces) then Minas Tirith would not have triumphed on the Fields of Pelennor. How could they, with Sauron's army and Saruman's from opposite sides? Even though the Riders wiped out loads at Helms Deep.
Theoden's army destroyed Saruman's forces at Helm's Deep. The only thing the Ents did was isolate Saruman. Even if the Ents weren't there, Saruman would still have lost his army. But he would have been safe in Isengard with the Palantir. Still, he wouldn't be able to go anywhere as Theoden would probably leave some of his troops around Isengard to prevent Saruman from fleeing.
Of course, one can argue that it wouldn't be possible for Aragorn to look into the Palantir and take the Paths of the Dead. But it is likely that Theoden's army would conquer Isengard : Gandalf the White would be able to break the walls of Isengard and Theoden's army would swarm in, and we would get a repeat of the Voice of Saruman chapter of LOTR, and Pippin would grab the Palantir. The palantir is a critical object : Pippin looked into it, and Sauron was decieved. Also Aragorn used it to show himself to Sauron and as such cause the battle of the Pelennor fields... it's hard to say if the Palantir didn't fall in the hands of Gandalf and Co, what would have happened. Sauron wouldn't have attacked MT so early, probably. We don't know what Aragorn would have done, take the paths of the dead or go with Gandalf to MT and claim his kingship. The result of the battle of the pelennor fields would be the same, probably, because Theoden still had his cavalry. After their victory : the march to the Morannon to distract the Eye. Hence : the end result is the same ---> Sauron defeated.
So anyway, with or without the Ents, Theoden would still have his cavalry to go to Pelennor fields and turn the tide there. One must keep in mind that Sauron cannot be defeated by arms anyway, only by destroying the Ring. The Pelennor fields and the battle at the Morannon gave Frodo the time he needed to complete his task. Saruman would only have the possibility to influence this string of events if he had his Palantir : he could use it to warn Sauron or at least give Sauron vital information which could prevent Frodo's mission from succeeding. But Sauron would still think that the Ring would go to Minas Tirith, so i am inclined to think that Frodo still'd have a chance to destroy the Ring. But this scenario is doubtful of course : after Helm's Deep, Gandalf would be wanting to deal with Saruman, so Isengard would have to be taken.
In fact, don't forget that Gandalf also showed himself in Fangorn and spoke to Treebeard, and Gandalf knew what happened to Pippin and Merry. Gandalf probably foresaw the Ents to enter the attack and break Isengard. So it is a bit hard to imagine having no Ents involved. Gandalf had it all worked out.
The Ents just made the taking of Isengard look easy, and as such came in very handy. Human forces would need more time and work, and they didn't have time for that, as MT was about to be attacked. But it is important to see that only if Frodo fails his mission, then Sauron will not be defeated. Even if the entire Middle-Earth falls under Sauron's red-eye banner, but Frodo manages to throw the Ring into the Orodruin, then Sauron is defeated (although this victory over Sauron would be very Pyrrhic...).
Although Tolkien had many influences for TLOTR, and many better and more profound than this, Tolkien's service in World War I had a large effect on the battle and war parts of the story. I think Tolkien might have been symbolizing America in WW1 with the Ents. America was skeptical about going into the war because they did not want to get involved for their own peace, but after they saw what needed to be done through certain turn of events, the joined the allied forces, which undoubtedbly helped defeat the German and Austria-Hungarian forces.
Some elements of the story he based on his (very short) war experience, like Samwise Gamgee, the Death Marches, but in the introductory to LOTR i believe JRRT himself says the world wars did not influence the story itself. JRRT himself wrote he loathed allegory. It is very easy to compare LOTR with WW1, even easier to compare with WW2, of course, but as i see it after reading the introductory, i believe JRRT only incorporated personal experiences of the war in his books, not politics or history.
I believe with the attack of the Ents on Isengard, JRRT was referring to nature striking back at technology. JRRT saw the place where he grew up being thwarted by technology, nature being replaced by technology. Maybe with the attack of the Ents on Isengard, JRRT was showing how much he disliked the fact that there was no more place for nature in England, that he made nature take revenge on technology.
Give up the Halfring, she-elf...