LaTeX is a free format and everybody may change the definitions and settings to his or her own needs (and is invited to provide suggestions/improvements as well).
LaTeX packages and classes orbit around the (La)TeX core and try to add features not already available yet (or do it differently, perhaps even better) than have been done before.
If a package macro interface is changed this is most likely done to choose better names or it's a major improvement (such as bugfixes) and not just because someone want to play around with names.
Even if the LaTeX Beginner's Guide 2011 suggests a now deprecated (???) usage of a particular package, this is a rather a very minor change, I expect 95% of the guide content to be fully functional nowadays.
Most packages do not change the design completely but add new features, which does not break other ones. I have published a few packages as well and I am constantly improving them (at least I hope doing so) and recognized that my earlier versions weren't designed best.
Finally, LaTeX packages and classes evolve -- this is a natural law of a free format.