They're hard to learn but from a logic and communicative standpoint, modal grammar is an incredibly smart thing to use.
In what sense? In my opinion English has done pretty well without all the distinctions made by other languages. Speakers are (usually) bright enough to infer any missing information from context. It's like the gradual disappearance of the subjunctive mood from English - we just don't need it because people realise from the context that the sentence is subjunctive.
I can't see any benefit in using a different form of the article simply because you're, for example, using a preposition. Why bother changing "ein" to "einen" in sentences like "Ich habe einen Bruder"? It adds nothing to the sentence and is just another thing to think about.