In most cases the public modules (like glsl-perturb-normal) exist so you can get things up and running quickly. After you're happy with the results, if you've benchmarked and found the shaders are causing performance issues, it is trivial to fork the modules and tweak things by hand, or just inline functions as you see fit.

Anyways, thanks for the comments. Modular programming has a lot of merits, but it isn't for everybody. :)