Phonetic spelling is useless when you’ve got more than one dialect. For instance, consider the word ‘castle’. Is this [kæsɫ̩], or [kɑːsl̩], or [kɑːsu] (as it is for me), or something else entirely? Having an orthography which is not purely phonetic avoids this problem.
Huh, what a coincidence — I just re-read Thud! a couple of weeks ago! An excellent illustration of my point that phonetic spelling is highly dialect-dependent.
(Oh, and don’t worry; I’m a native English speaker, and I can’t understand Pratchett’s gargoyles either.)
Not quite. No script, even the IPA, is perfectly phonetic. Natural languages deviate in so many ways over time that this is the only possible eventuality.
Language is unconscious and you won't understand how it works until you let go of a lot of false culturally-programmed notions, and then actually study it by engaging with the work of linguists.