I'm not sure how many languages this works well for but it certainly doesn't work with C. You won't learn the important distinctions between implementation defined, unspecified and undefined behavior. And reading a C standard library implementation will not really teach you to write normal idiomatic C code. You will also not learn the strict discipline required to avoid doing things unless you absolutely know with certainty that they are either strictly conforming or defined for your particular set of target platforms (usually much harder to answer this second question which is why when writing C you should avoid having to).

That being said, for Erlang, I think this is a good idea. I learned a lot when reading the standard library.