As an example, b in English is called an unaspirated voiced consonant, which means you vibrate your vocal folds, but send only a little breath when you sound it. You can compare it with p, its aspirated voiceless counterpart, for which you do not vibrate the vocal folds (voiceless), but sends a strong breath through your month (aspirated). Chinese does not have most voiced consonants present in English, so Pinyin naively uses b (also d and some others) to represent voiceless aspirated consonants, which does not sound like a b, but more like p in “spade”. This results in most of the foreigners learning Chinese with Pinyin bringing English pronunciation when they speak, and makes them sound way too stiff and thus awkward.
Wade-Giles, in comparison, is much more systematic, and does a much better job hinting speakers with European (especially Germanic language) background to sound more correctly. Bopomofo does not have this problem, as it basically invents a new set of scripts. Pinyin probably lets you pick up casual speaking Chinese most quickly, but beyond that, it’s a curse.
[Edit]: Voice “folds” not cords, sorry. Also fixed some typos.
There are two bilabial stop consonants, called p and b, in many different languages, with different sounds, e.g.
Mandarin English French
unaspirated voiced - b b
aspirated voiced - - -
unaspirated unvoiced b - p
aspirated unvoiced p p -
though there is, as you point out, an unaspirated unvoiced allophone of p in "spade". Other stops (t, d, k, g) follow a similar pattern.English speakers using English p and b in Mandarin will still be understood even if their b sounds foreign, but French speakers might not be, because French p sounds like a Mandarin b.
The only change I'd like is replacing Pinyin -ong with -ung.
Pinyin is pretty well-adapted for entering Chinese, but I think that goal is at odds with being easy to pronounce. I'm not sure that having one ISO standard for both purposes is a good idea, although if China ever starts to export more culture, maybe everyone will eventually learn how to pronounce Hanyu Pinyin.
Still, I agree with your main point. The Latin alphabet is not used like the IPA by most languages, even English has not preserved all the original Latin sounds as used by the Romans. And there's even precedent for some of the sound choices made in Pinyin that would seem completely alien to English speakers. For example "c" is used in all the Slavic Latin alphabets (e.g. Polish, Czech, etc) for the unaspirated version of the Pinyin "c" sound. German and Pinyin use the letter "z" for the same sound, etc.
In any case, as a person who has a decent pronunciation in Chinese but lacking the ability to fully read/write, Pinyin has been absolutely great for being able to type Chinese informally. I can communicate almost anything I can say, and that's truly wonderful. Part of that is the smart prediction software. But Pinyin has a big part in it too: it's not difficult at all to sound out a word and figure out how to spell it in Pinyin. That's the essential feature that Chinese characters lack, and Pinyin nails that essential quality as far as I can tell.
I should make it clear I don’t think Pinyin is a bad system, only that it does not map sounds to European languages well. And that is fine, as least for its original purpose. Contrary to common belief, Pinyin was not developed to better educate illiterate people (this does not even make sense if you think carefully), but as the next step after letter simplification (what we know as Simplified Chinese today) towards Chinese Romanisation. The goal was not to represent sounds of Chinese characters, but to outright replace them. The actual reason behind using (for example) b and p instead of p and p' is exactly this—they are optimising for ease to write, not sound accuracy.
And in regard to making it easy for people not prolific with Chinese letters to read, be them foreigners or simply illiterate, a phonetic transcription system is indeed important. But although having a standard phonetic transcription is key to the jump in literacy in China, I would argue the same can be achieved with any decent system, and a few of them are already around by the time they started working on Pinyin.
All in all, while Pinyin is good for its original purpose, it achieved its current status not because of academical reasons, but political ones. The Chinese Communists government had always have a habit ignoring existing systems and inventing their own. :p
Pinyin does a good job in providing a fairly consistent way to present Chinese sounds using Latin alphabet. It's also the workable way to input Chinese characters on a computer keyboard. It works, it is consistent, and it is now ubiquitous in China.
1. unaspirated unvoiced (av)
2. unaspirated voiced (aV)
3. aspirated unvoiced (Av)
4. aspirated voiced (AV).
German and I think English have 2/aV (b, d, g) and 3/Av (p, t, k), the diagonal so to speak (for the beginning of a syllable).
Mandarin has 1/av and 3/Av. So, 3/Av is pronounced the same - and Pinyin renders those as (p, t, k), as in German/English, so that's actually nice. Wade Giles renders them as (p', t', k'), indicating the aspiration.
German and English do not have 1/av (at the beginning of a syllable). Pinyin then chose to render them as (b, d, g), correctly indicating the unaspirated character, but tempting German/English etc. speakers to voice them (wrongly). Wade-Giles render them as (p, t, k), tempting German/English speakers to aspirate them (wrongly).
No easy solution. Plus, the apostrophes are so often ignored (e.g. Pinyin Taibei = Wade-Giles T'aipei).
I find Wade-Giles ch'in more evocative than the Pinyin qin, and similarly hsüan vs xuan, and even tzŭ vs zi and tzʻŭ vs ci. Wade-Giles you basically have to learn not to ignore the apostrophe, Pinyin you have to learn most of the initials. Though the Wade-Giles jih vs Pinyin ri is a bit of a toss-up.
For finals, I find Wade-Giles yü more consistent than Pinyin yu, but Wade-Giles to less descriptive than Pinyin duo. But then, it's shorter.
So, yeah, "arguably" :-)