i almost don’t need to reply here because cranky laowai is saying everything that needs to be said. but lest someone think he is alone in his opinions 
rascal, saying taiwanese is a dialect of mandarin is like saying italian is a dialect of spanish. they have common roots but are completely different languages. you might consider both to be dialects of “chinese” but not one to be the dialect of the other.
remember that mandarn itself had no written form until 100 years ago, before this there was only literary written chinese which was used throughout china. that taiwanese still has no written form does not make it less of a language. it is simply a language that because of Taiwan’s history has never acquired a commonly accepted written form.
an on-line dictionary search shows this for dialect:
1 a : a regional variety of language distinguished by features of vocabulary, grammar, and pronunciation from other regional varieties and constituting together with them a single language b : one of two or more cognate languages c : a variety of a language used by the members of a group d : a variety of language whose identity is fixed by a factor other than geography (as social class)
under b you could call taiwanese and mandarin dialects but not one of the other. your german dialects are cleraly covered under one. but that is not the case for minnan and mandarin. as cranky rightly points out taiwanese is one dialect of southern min.
cl also points out very well that spoken mandarin is no more difficult than other languages, the characters are though! “just using the characters” for taiwanese raises a very large number of complicated questions, not least of which is having to learn the character associations for various commonly used words! this would have to be LEARNED and believe me it would be far, far harder than learning a romanization. in fact it would be impossible. there simply ARE no commonly accepted characters for much of Taiwanese an entire system would have to be created first! on the other hand, a romanization is very feasible and quite easy to learn. you need to do your homework on this point spack. perhaps if people went through life singing ktv songs it would work, but i think that would have a number of other negative effects you havent quite thought through 
cantonese does have a commonly accepted written form using characters, but this is not something that someone just pulled out of a hat, is has taken form over time. it also has to be learned, just as cantonese must learn baihuawen in order to read newspapers etc in that langauge.
as for the value of learning one’s mother tongue, this depends on your viewpoint. consider that taiwanese education has been surpressed for political reasons for 100 years. kids used to get hit if they spoke taiwanese at school. they were taught to think it was low class. the simple fact however is that it is the mother tongue of the majority of people on the island. that it be taught for a few measly hours a week in school in most of the world would be decried as a gross violation of human rights, but in taiwan it is considered “too much of a burden” for children to learn it in favor of Chinese and even ENGLISH god help me. i wonder what the malays would do if the chinese and the indians came over and took over for 50 years each not allowing malay to even be spoken in school. if that were the case your comparison might be more valid, Omniloquacious.
i’ve gone on a bit but i’ve given this one a bit of thought and research over the years 