@pxeger It can be useful, but a) not all languages have it, so that gets back to the "hard for speakers of non-English languages to pronounce," and b) it doesn't have a natural letter to represent it in ASCII.
@DLosc (which I realize is slightly contradictory to my earlier suggestion about i and u as semivowels, but at least those sounds are phonologically very similar to their vowel sounds)
@RedwolfPrograms I rather like the idea of having a particle before each noun phrase that conveys number and case. We've already had a suggestion of o for singular and i for plural--those could just be converted to the singular/plural of a specific case, with other variants added for different cases.