Ahhh! Back to guess work. How I miss threads like this...Such as "What would have happened to Gandalf?" or "Boromir: What would have happened if he didn't die?" (the last one I started
).
Like I said, this would all be speculation. But sometimes, the specualting could be the best part of the discussions.
There are plenty of things that could have happened.
But the most likely would be that they still would have ended up in Fangorn and meeting Gandalf anyway as it wasn't Aragorn and Co. who rescued them, but (without their knowledge until later days) the Rohirrim. So, all in all, I think that part of the story wouldn't have changed. They meet Gandalf, send the Ents to war on Isengard, Pippin sees into the Palantir and goes to Minas Tirith and Merry goes off back to Dunharrow and the to the Battle of the Pellenor Fields.
The only plot change would have been Aragorn, Gimli and Legolas helping to find another way (not through Shelob's lair) into Mordor.
Well, that's my belief anyway.