I didn't intend to imply all vowels can be dropped or wild-carded in English. As far as schwa, if we do refactor English, we should probably stick to existing "normal" letters rather than introduce symbols such as an explicit schwa. I agree this risks either ambiguity, or some potentially non-obvious rules to remember.
As it is, I feel sorry for non-native English learners; English's spelling is a mess. The only saving grace is that being semi-phonetic means one can often make a decent guess.
But vowels do matter in English. Consider the following words that differ only in the vowel they use: bat, bait, bit, bot, bite, boat, boot, butt, bet, beat, bout. Furthermore, all of those words refer to very different things. Semitic languages are based on a triconsonantal scheme, where three consonants define a root, and the choice of vowels implies inflection. Dropping the vowels in Semitic languages does not impair meaning all that much.