I disagree. I tried learning Catalyst for Perl a few years back. You could swap almost any component in it, and the tutorial explained how to. Being new to the whole concept of a web framework, I couldn't actually work out what the framework was doing, as it just seemed like a collection of libraries for various tasks. Learning Django after that and it all made a lot more sense. You can swap parts out, but it makes more sense, especially in the begining to use the sensible defaults provided. They are probably more coupled than they could be, but it works well, and the parts play nicely.

JavaScript frameworks seem to be another matter.