We've had a few thousand years to figure out decent ways to authenticate paper-based documents. Presumably, one of the biggest implications of a paper-based solution is that you get some level of contestability. In the case of a signature, you get penmanship (at least to a certain degree of expert evaluation); for currency, you get the paper type, watermarking, holograms, etc.
You have very little of that in the process described by the OP. The accountability is "email is secure, and you know the person's voice." That's fine if you implicitly trust your contracting counterpart (heck, then the contract has little purpose). But it does little if they later recant or refute the signature. In the end, the most important aspect: It needs to be tested in a court of law before I'd even consider it.
I'm not sure what the "right" solution is in the digital age, but my hunch is that it will involve some sort of PK Crypto. Perhaps we're not too far from having NFC tags embedded in our body for precisely this purpose.
