Magus' name would actually be translated as Maou, but if you say that in English, it doesn't have a particularly pleasing or imposing sound to it. Plus the meaning of the name is lost. 'Magus' has a much stronger sound and keeps relatively close the original 'feel' of the name.
Exactly. It's all about what the name implies. Purists who say 'the original must be kept' because they know the language are being a little arrogant, actually. They know it, they like it, and think it must be kept so for everyone, but fail to realise that the what is important in a name, a word, and a line of words, is the effect they bring across. Now, an identical phrase in two cultures can be vastly different in meaning: praise in one can be insult in another. As such, to leave it the same for both cultures is removing the most important aspect: the meaning. If one were to leave the name 'Maou', what you've got is a shell of a thing - the meaning has been left behind.
You're absolutely right in what you're saying. What's important in a word or name is the mental image it brings across. To English speakers, Maou is nothing. It's a name. It's M-A-O-U, and nothing more. Magus, on the other hand, is historically grounded in the culture to mean a sorcerer. The meaning has been transferred. So that's what it comes down to: do you wish to transfer a shell, or a meaning? Purists are usually for the former, wheras the rest are for the latter.
Now, you must remember, I'm a bit prone to being purist myself in several matters, so I know how it feels to want to keep the original. In such things as Lord of the Rings, for example, I could point out various aspects of the movie that were changed going from the Tolkien sub-culture to the more wide-based movie-watching culture. To return the Elves at Helm's Deep; to make Faramir indecisive; to make Legolas' hair gold. Well, the purist in me rebels at these things. The Last Alliance was just that; the Elves of Lothlorien had their own borders to defend during the war; Faramir was not tempted by the ring at all; and Legolas... the more knowledgeable of fans would know him to be, with a high degree of probability, a Telerian elf, and that's dark-haired - he would not have gold hair unless there was some Vanyar blood in him. But on the flip-side, when watching the movies, I considered... is this really that relevant? Does it really matter so greatly? The meaning has been conveyed, so do I care? In the end, it worked. For a second thing, there's Homer. Now, Troy for the most part split with the Iliad, yet still professed to be based on it. Agamemnon was not power-hungery; Menelaos was by no means boorish; Hektor was a bit of a coward; Akhilleus was far more thoughtful and never loved Briseis; Aias did not die there; and the number of heroes, from Diomedes to Sarpedon, that were removed is staggering. You see, even by the way I write those names, I've got a purist streak. But the fact is... yes, I would have liked the movie to be more exact to the Iliad (which, actually, story-wise, it wasn't at all: it began too early, and ended too late, and barely focussed at all on the main purpose of the epic, which was the anger of Akhilleus.) But for a movie-going audience... it wouldn't have worked right. There are certain admissions that must be made (though, even to this day... I loved the armour in Troy, but so wished they had kept the chariot race from the epic in it - that would have been a thing to see!) Moreover, just for Greek things in general... who says Alexandros for Alexander? Or Platon for Plato? It's be stupid to be overzealous in the applying of the actual names.
Anyway, though, the point is, not everything translates over rightly, and even in the West things are changed. It's even more so in things across such varied cultures.
