I have always tried to make sure that the entries in LPD are based on objective observation of usage as well as on my native-speaker intuitions (and, in some cases and to a minor extent, on what other reference books assert). But I too am human, and it is possible that I have sometimes got things wrong.
I am aware, for example, that many people use a laxer, ɪ-like quality when a stem ending in “i” is combined with a consonantal inflectional ending, as in studied, studies. (Personally, I have no such alternation, since like DJ I have phonetic ɪ throughout.) Yet (in my experience) such speakers often maintain, nevertheless, that studied does not thereby become a homophone of studded, even if the latter does not rhyme with (nonrhotic) juddered. So that’s why I allow for a three-way contrast: studied ˈstʌdid — studded ˈstʌdɪd — juddered ˈdʒʌdəd. (There are also people who do rhyme the latter two, and have ˈstʌdid — ˈstʌdəd — ˈdʒʌdəd, while rhotic speakers obviously distinguish juddered from the others. For me personally, on the other hand, studied and studded are homophones.)
If I had introduced an extra symbol ᵻ, giving study ˈstʌdi, studied ˈstʌdɪd, studded ˈstʌdᵻd, juddered ˈdʒʌdəd, I am sure people would have regarded that as too complicated. There are never four such weak vowels in contrast in the same environment. (Notice, too, how ODP, which does use the symbol ᵻ, sometimes gets it wrong, as when armistice is transcribed ˈɑːmᵻstɪs instead of the correct ˈɑːmᵻstᵻs, as demonstrated by Australian ˈaːməstəs.)
The same arguments apply, mutatis mutandis, to the -s ending: taxi — taxis, compare taxes, taxers.
I also leave the i transcription unchanged when suffixes such as -ness (shabbiness, holiness) are added. The suffixes which for many speakers — it seems to me — clearly do affect final i by changing it either to ɪ or all the way to ə are those involving the lateral: -ful, -less, -ly. So beautiful has -əf- or more conservatively -ɪf-; merciless likewise; and angrily ends in -əli or -ɪli. This is seen most dramatically when steadily and readily are pronounced with lateral release plus syllabic l̩, for which the rule of syllabic consonant formation requires a schwa input. As I see it, the synchronic phonological derivation of this variant has to be ˈstedili → ˈstedəli → ˈstedl̩i. Likewise penniless → ˈpenl̩əs.
Why do I write -ɪ-, -ə- in polythene but -i- in polytheism? Not because of some general principle, but because it is my impression that people treat them differently. Am I wrong? Similarly with various other supposed “inconsistencies”. Ultimately, every word has its own pronunciation.
Still more on this tomorrow.