Take "simpatia" and "ânsia". In the first case, the last "ia" is not a diphthong. In the second it is and you need the diacritic over the a to make it the tonic syllable (Spanish is more clever in that the general case is that the "ia" ending is taken to be a diphthong unless you put an acute sign over the i - ía).
But, when diving further into Russian, I soon found out that they do weird things like pronouncing a G as a V (его - his).
So, my conclusion is that even if you have the perfect alphabet, it will soon become obsolete.