Its an interesting discourse. I'm not against people designing alternative solutions when they find the defacto standard is lacking (jQuery UI). The benefit of having a standard "batteries included" solution can't be denied either, look at the ease of adoption of Python or Rails. I think the biggest concern isn't that jQuery Tools offers a different library, its that it goes about doing it in non-jQuery ways.
I think that if jQuery Tools works for you fine, but ultimately this functionality should be subsumed within jQuery UI so that there is a single repository for UI elements.
jQuery UI has done a lot of work in standardizing and componentizing their library, this functionality should be integrated with the UI canon. Then when I want to have functionality only offered by jQuery UI and functionality only offered by jQuery Tools I can use them in harmony, and that includes the nice extras like themerolling.
I'm doing tabs with plain html and CSS; am I an idiot who is missing something, or am I some kind of pioneer who owes the world a few alistapart articles? ;-)
Actually the comments are more interesting and noteworthy than the article itself.
Also the author of jQuery Tools responds in the comments.