In Latin Language: Is the difference between long and short vowels really important?

The difference between long and short vowels (quantity/duration) is always highlighted when studying Latin. But I wonder, if that difference is so important even to the extent of ambiguity, how do Ecclesiastical Latin speakers understand each other. I know that Catholic churchmen speaking Latin don't make that distinction, but they pronounce consonants in a different fashion (you know ce-ci-cy, ge-gi-gy, ti-, et al.). Anyway if somebody says "chelum" I'll understand "caelum", I don't think I would get lost in the world. But what about in the case vowel?