In my personal experience, most companies have just as much information available over their wireless network as they do over the network cables.
Most of the time, they seem to just grab a wireless router from the local electronics store and plug it into the network so they can put their portable toys on the internet.
On the other end of things, there are also those businesses that move almost all their data over the wireless since most of their computers are laptops and smartphones.
Either way, wireless networks represent a significant vulnerability no matter how well a business segregates and manages their network. Practicing good security is always a worthwhile investment.