Good way to put it!
Shel,
From a hearing person's vantage point, as spoken language is the medium of communication that conveys the most information quickly. The written language compliments it but it is more rigid and inflexible and takes longer to get across. Put differently, there is a whole lot more give and take with the spoken language that the written one can't match no matter what. One doesn't have to be so grammatically accurate as the communication protocol is continuous and active for the participants. It is much more difficult to achieve that in the written form.
Take "IM" for instance, nobody text messages the way one would expect in the pure written form simply because it is way too slow and cumbersome. I don't use "IM" but I have seen enough of it to know that it takes on the spoken language "tempo" and "lingo" and it shortens many words and expressions to be similar to spoken language in terms of getting the point across quickly. No wonder it is so popular...
Still for all that, it doesn't easily convey the additional information that voice can with spoken language (think irony, sarcasm, humor) without adding extra verbiage.
For a comparison, sign is the same as spoken language in that sense of give and take. The problem is for a deaf person who can't use spoken language, is the fact that the written language is not the same as spoken language in its myriad forms as R2D2 was pointing out.
To make this even clearer, spoken languages were developed long before written language ever gotten off the ground. Obviously, this would not have been the case if it weren't a better means communication for the hearing. Heck, one doesn't has to be formally educated either to use it!!! On the other hand, this is not the case with written languages.