Hi Thomas,
I'm not an expert on Dojo, and there are many other pure Javascript frameworks out there like jQuery, MochiKit and Mootools that readers might be considering too, all of which have their individual strengths. Before trying to persuade anyone to use P & S over and above any of these others, I strongly urge anyone not using a helper library to start doing so! Whichever you pick, you'll get 80% of the advantage over trying to do everrything yourself. Now on to the remaining 20%...
Speaking personally, I chose to write about P & S because they were the libraries that I was using in production, and I like to know what I'm talking about! Seriously, I try to write from experience, and I picked on P & S as a bookj topic because I felt that using them had changed the way I code, and I wanted to share that new perspective.
I've found Prototype to be extremely good at speeding up my coding - Sam really 'got' JavaScript before many of us did, and Prototype allows you to write code that maked good use of the language's key features (object literals, first-class functions, closures), and extends the core language in a natural and pleasing way.
So, having worked with P & S for a couple of years, I'm a better JS coder now, even when I'm not using those libraries. You might be able to learn these lessons from John Resig's jQuery too - he seems to have 'got' JavaScript pretty much too, but I can only speak of the path that I took. There's plenty in the book about how these core language features work, as well as how Prototype builds on them, of course - see chapters 8-10.
Scriptaculous too takes a sensible approach to the UI, using JS to add behaviour to the DOM, working _with_ CSS, HTML, DOM etc. rather than trying to go over-the-top in OO terms.
There's a very active community around P & S, at the rails-spinoffs google group. (Plenty of rails developers are there, but also PHP,
Java, .NET and other sorts, don't be mislead by the name.)
HTH
Dave