Hmm. But English is made up of phonemes-- there's just a lot of borrowing from other languages, which is why it's not consistent.

One of the reasons Deaf speakers of ASL have trouble learning written English (and this applies over other languages) is that they conceptually have trouble with the phonemes. (This is why if we needed to teach my daughter a signed language it would have been cued speech.) One of the big challenges in terms of getting Deaf people access to proper employment, etc. is bridging that gap. A 'phoneme'-based language would help in that respect in a way an abstract one wouldn't.