A common written version means a common language, even if the spoken versions are unintelligible.
As for tones, the fewer, the better, so four tones is considerably simpler and more efficient than twelve, particularly for those learning Mandarin as a second languages. Languages tend to grow simpler with time.
French and English differ in both spoken and written forms; they are truly separate languages, mutually unintelligible in both media. However, they do share a lot of vocabulary, and if there were considerably more commonality of vocabulary and similarities in grammar, they might be dialects of each other rather than independent languages. If one compares modern English to English before the Normans started interfering with it, it becomes apparent just how close English came to such a merger.
Today, an Anglophone cannot understand a French newspaper, and vice versa. However, a speaker of Cantonese can understand a newspaper written by Mandarins, and vice versa. Indeed, the written language allows the two to communicate even if they cannot understand each other's speech. The disparity in speech is one of the many disadvantages to having a written language that is uncorrelated with pronunciation.
<<A common written version means a common language, even if the spoken versions are unintelligible.>>
Mxsmanic, no amount of bullshit on your part will change the fact that Mandarin, Cantonese, etc. are separate languages. And as I have stated before, human languages are *spoken*, first and foremost. Writing is a secondary characteristic of languages. When speech tells you there are multiple languages, and writing tells you that there is only one language, speech takes precedent.
I ask you again: Say that English and French decided tomorrow to institute a common logographic alphabet - for example, a hexagon represents both "door" and "porte"; a diagonal line represents both "foot" and "pied". Would English and French then cease to exist as separate languages?
I have had a few friends say I do not speak Chinese, in reference to Mandarin. Both were from Taiwan. I do not mind it particularly because I just do not care. I do not like how Mandarin sounds anyway.
Below is a reiteration of what Lazar said. Read if you are bored.
Common orthography does not mean the same language. Writing is a representation of an idea. Different languages choose different sounds to represent the same idea, and so is the agreed upon symbol or combination of symbols to represent an idea. However, the agreed upon meaning of a symbol can change without regard to the langauge. Like Lazar said, just because two languages share the same writing, does not mean they are the same language. And to further his example, suppose tomorrow, all the languages of the world decided to agree that the number 1 would be written as a horizontal line " - " like that instead of being spelt "one" in English, or "un" in French, etc... and the word for "middle" or "center" would be a rectangle with its long side as the horizontal and a line drawn through it vertically. Would all the languages become one language with many "dialects"? The symbols I described were Chinese characters representing "1" and "middle."
While there are true dialectical variations of Mandarin, such as the north-east rolling "r", Mandarin and Chinese are two different languages. Both do belong in the same over arching language class. I would say they are like French and Spanish. Both French and Spanish are classified as Romance languages, and they are spoken in close proximity to each other, they share similar words, however they are not the same language. A Frenchman cannot understand a Spaniard unless he has taken Spanish, and vice versa. They may be able to guess at a few words in the others language, but they do not understand each other. Same with Cantonese and Mandarin. Speakers of both languages can guess at what the other is saying sometimes, because the words sound similar, but not always. There are words in Cantonese and Mandarin that are totally different from each other. Also, there are grammar considerations. Cantonese is always SOV, like English, however, Mandarin occasionally allows for SOV constructions.
I can only repeat what I have said loads and loads of times: a distinction between a "dialect" and a "separate language" is a relative one. It's all the matter of tradition, history, culture, politics etc. Swedish and Norwegian, Russian and Belorussian, Czech and Slovak, etc, etc. are much closer to each other than some "dialects" of Chinese. Hochdeutch and Nederlandisch are different languages, while Hochdeutsch and Schweitz Deutsch are "dialects" of the same language. Thats says a lot... It's all relative. Samogitians, by the way, also have their own "separate language" and they hate when it is being called "a dialect of Lithuanian".
Talking about the Samogitians and the Chinese, here is one famous joke that came to my mind: once Samogitians decided to wage the war with China. The Samogitian chief is calling Chairman Mao and says: we will fight you. Chairman Mao asks: how many troops will you have. The Samogitian guy says: we will send you 100 men. Chairamn Mao says: we will send you 100 million. Then the Samogitian guy hangs up and says to his men: oh goodness me! We cannot fight them. Can you imagine - they are sending 100 million soldiers. Were will we burry them?