Consider that the peak throughput of the PCI bus is 132 MB/s. 802.11G wireless pushes 54 Mb/s. That's bytes vs. bits, mind you... And PCI-X runs at 1,066 MB/s. So yes, speed would be KILLED. (
Source)
Wireless prevails where people do not want to or cannot run cables. Inside a box is the perfect place to run cables.
Even if wireless and wired speeds were somehow equaled, consider the added cost of implementation, the possible security implications, compatability issues, and greater suscpetibility to interference.