I'm not sure where you're getting this. Antorell in particular is pretty incompetent, but this is the case even relative to other wizards; he just happens to recur a lot. The wizards are able to assassinate a king (with help), kidnap a subsequent king, and then ultimately create a large long-term problem for a third king and his family. They're effective at causing trouble, they just don't win in the end.
They are effective at causing trouble, but they manage to do so despite being pretty heavily disadvantaged magic-wise compared to the protagonists. Zemenar was supposedly a very exceptional wizard, but when he actually takes the stage his abilities never seem particularly impressive. When I first read the first book, I was expecting him to turn out to be subservient to some Bigger Bad, because he just didn't seem as threatening as I expected from a primary antagonist.
There are certainly stories with villains who accomplish less. In plenty of kids' media, the antagonists will be implicitly very powerful, but never accomplish much at all. But in the Enchanted Forest series, I felt like it was the reverse, that pretty much everything the wizards pulled off, they did with the odds stacked against them.
The word didn't work to start out. That was the more portable alternative to the buckets of water that had to be developed deliberately by Telemain; wizards aren't just inherently vulnerable to the word "argelfraster".
Even so, I felt like weaknesses of that magnitude really served to trivialize them as antagonists. The fact that they could be defeated with water with lemon and soap was already a demeaning vulnerability which took away a lot of the tension from any encounters with them. I thought that it would have improved their stock as villains if they researched some defense, and at a critical point where the protagonists tried to melt them with water, they revealed, "Hah, that will never work again!" But instead, the protagonists acquired an even more convenient method to defeat them, while the wizards retained their old vulnerability.
I liked the protagonists, but I got the impression that Patricia Wrede didn't like her own antagonists; it wasn't enough for them to lose, they had to be demeaned and trodden on.
Follow-up To: On the Care and Feeding of Young Rationalists
Related on OB: Formative Youth
Eliezer suspects he may have chosen an altruistic life because of Thundercats.
Nominull thinks his path to truth-seeking might have been lit by Asimov's Robot stories.
PhilGoetz suggests that Ender's Game has warped the psyches of many intelligent people.
For good or ill, we seem to agree that fiction strongly influences the way we grow up, and the people we come to be.
So for those of us with the tremendous task of bringing new sentience into the world, it seems sensible to spend some time thinking about what fictions our charges will be exposed to.
The natural counter-part to this question is, of course, are there any particular fictions, or types of fiction, to which we should avoid exposing our children?
Again, this is a pattern we see more commonly in the religious community -- and the rest of us tend to look on and laugh at the prudery on display. Still, the general idea doesn't seem to be something we can reject out of hand. So far as we can tell, all (currently existing) minds are vulnerable to being hacked, young minds more than others. If we determine that a particular piece of fiction, or a particular kind of fiction, tends to reliably and destructively hack vulnerable minds, that seems a disproportionate consequence for pulling the wrong book off the shelf.
So, what books, what films, what stories would you say affected your childhood for the better? What stories do you wish you had encountered earlier? If there are any members of the Bardic Conspiracy present, what sorts of stories should we start telling? Finally, what stories (if any) should young minds not encounter until they have developed some additional robustness?
ETA: If there are particular stories which you think the (adult) members of the community would benefit from, please feel free to share these as well.
ETA2: My wildly optimistic best-case scenario for this post would be someone actually writing a rationalist children's story in the comments thread.
ETA3: On second thought, this edit has become its own post.