...they're not competent to design cover art, except those with a visual design background. That's a job for an art director with an artist. The publisher's goals for covers isn't primarly to be nice or good art or even to reflect the content of the book accurately, it's strictly art in the service of marketing. Colours and fonts are chosen for visual impact, art is suggesting what the publisher wants to suggest to the readers and so on. In the case of a series, they try to create something akin to brand recognition. Cover art isn't much different to a Tide box or Coke can. The publisher will change it along the way only when they realize the style no longer does it job as "branding", or to reach a new audience (Erikson's books were repackaged that way. They dropped the more classic fantasy style for something more modern and more realistic and generally darker. It looks that they adjusted the style to the demographics of Erikson's readers - by their looks I'd say they are aiming those covers at the 18 to 30 y.o. males). Other publishers (with a less varied audience than Tor that publish stuff that appeals to very different audiences in a broad genre) will rather seek to make the publishing House or its collections the brand. Tor rather designs a branding for each distinct series, not always aimed at the same people.

Jordan's side had an input, but he usually let Harriet deal with the "packaging" of the books (she named most of the chapters and chose the icons, she is the one who hired a designer to draw the chapter icons following her ideas, she wrote all the cover blurbs and she's mostly the one involved with Tor's art dept. for the cover art) and she still does. Either Jordan wasn't interested, or more likely he just felt Harriet had more experience of Fantasy/SF publishing and knew better than him, had more distance to the work too. A writer would go with his personal tastes (which often enough may be quite poor) and his own vision, and that's not what professionals of marketing would choose to sell the books...

Sweet's covers are awful to the tastes of many (incl. me) but it's also instantly recognizable illustrations. The layout, the colours, the illustration are all very "1990s" (the 1990 version of the 1930-1940 pulp style) and outdated, but readers don't have any problem spotting a WOT book in store. That's obviously what Irene Gallo (art director at Tor) is asked to deliver for WOT. Just a look at the Tor output as a whole shows that their art dept. is able to create covers in all sort of styles, it's a conscious decision to stick to the tacky/old fashioned look for WOT. It was a conscious decision in the 1990s to market the series this way instead of presenting a more realistic vision of the content, and now it's a choice to stick to that "old fashion" look. Harriet explained it last year: they have no intention of changing the look until the series is completed. Afterward they'll come up with a new look and new artist(s) for future editions. But for now the Sweet look is like branding. Orbit dropped him and use the Wheel-serpent to achieve a similar effect (probably more appropriate to their market. The WOT look is a very American way to design books.) Readers not aware that a new book is out will spot them instantly in stores, or in advertising.

As for the mistakes on the ToM cover, it's normal. It's part of the process. Commercial artists like Sweet get their draft or concept art corrected or even refused and re oriented all the time (just look at some of Gallo's commentaries on the e-book art, it gives a fair idea of the creative process of designing a cover). Sweet's work starts months before the book is ready. He gets but a few pages and descriptions, and instructions from the art director as to the feeling or approach the publisher is looking for.