Rails is a DSL for building web apps. Ruby is a great language for building DSLs. Building up a DSL to meet your domain needs is a nice way to go. I don't think this is a problem at all, and very much in the spirit of both Ruby and Lisp (from which ruby takes a lot of inspiration).
I believe Ruby's open classes were also a reaction to the locked down libraries that were common to Java. They were impossible to extend and resulted in a rats nest of adapters, visitors, and every other pattern in the book. In Ruby the programmer is in charge, not the library authors. The ability to do 1.day.ago was not just cute, but revolutionary, and huge productivity boost for anyone coming from C, or Java, or C# - the predominate languages of the time.
I believe Ruby's open classes were also a reaction to the locked down libraries that were common to Java. They were impossible to extend and resulted in a rats nest of adapters, visitors, and every other pattern in the book. In Ruby the programmer is in charge, not the library authors. The ability to do 1.day.ago was not just cute, but revolutionary, and huge productivity boost for anyone coming from C, or Java, or C# - the predominate languages of the time.