Some words appear to violate this rule, but on closer inspection we see that they don't. Separate vowel symbols, if necessary. It doesn't happen often, but occasionally you'll come across a word in which a combination of vowel symbols causes ambiguity. Most people would probably write it as SIE ENS, with a dot or a dash in the middle. Without a separator, there would be two possible pronunciations of the word.
Unnecessary symbols cause clutter.
People who overanalyze tongue positions sometimes use unnecessary letters. Speechskript isn't a concrete lexicon of spellings meant to replace our richly diverse written language. It's a totally different construct. It's more like a fluid game that people play with a handful of simple phonetic rules. People who tend to pronounce words differently from others can be expected to spell their Speechskript words with similar differences.