Ok, she might talk, but they could always scare her into not talking. Kidnapping her is a surefire way to get half the Great Valley on your tail, which is exactly what Pterano hoped to avoid. (Why didn't he get banished for longer for the whole kidnapping thing?)
Also, what exactly was Pterano going to do,if the SOCF actually did work? I mean, I doubt Topsy for one would just make him the leader without a fight. Is he going to use the stone to get mind control powers? Even if he gave himself the abilities of Sharptooth from the first film, I doubt they'd just hand over control of the Great Valley to Pterano (unless he was hoping to get "seem wise" powers to make himself seem wise so that the Great Valley would make him the head honcho.)
I'm not even sure why they banished him to begin with. It was the herd's own fault that they followed him. It's not like there were kids following him. Everyone was of age.
Also, let's get real, what was Pterano supposed to do, fight off all of those Sharpteeth by himself? Did he mutiny or something and take command by force and that is why they were so mad at him? Seriously, I don't see what he could have done.
Yes, he has a big ego and yes many died, but it wasn't on purpose. He, like Cera in the first movie, may have tried to go an easy way and gone right into a Sharptooth ambush. You can also tell, based on his remorse after thinking Ducky dead that he does regret what had happened earlier (and not just because he got banished).
Still, back to kidnapping Ducky, that was deliberate and it would certainly get him in more trouble and certainly can't have been any better than just taking off and letting her go free.
Also, what is he gonna do, bring her back later, stone or not, and expect a warm welcome? On the other hand, if he was thinking of killing her, not likely, one would think the Great Valley would be really mad (I wonder, you don't think they would have a death penalty in the Great Valley for such a horrible act, do you?)