Wow, two posts showed up, while I was typing this, haha.
WiFi is useful for some things, tablets, remote audio devices, etc.
However, you must realize that WiFi is not a switched medium like when you connect something via Ethernet. (I say this, because I don't know ANYONE who actually uses a hub to connect devices anymore, switches have all but subsumed them.)
But why is this important?
Ethernet, natively, is a protocol that was designed for half duplex communication over a set of wires. By itself, it has no way to do full duplex communication between two hosts, because other nodes may be communicating, and it still is only one set of wires. It employs a method called CSMA/CD, which listens to the wire, and sees if it is clear to communicate, and if so, it sends its next packet. If the wire is talking, it waits a random amount of time, and then tries again to send the packet.
If two devices try to send at the same time, the network cards sense this (because of a doubling of voltage on the line), the cards immediately stop transmitting, and wait. This is known as a collision. It results in lost time that could have been used to transmit data, and therefore a loss in overall network efficiency.
Makers of Wired ethernet networks (particularly 3Com), realized that while this method worked well for the first 10 megabit ethernet networks, as networks were being tuned to go faster, the potential for collisions increased exponentially (and so it panned out this way), so a solution was made on both the side of the hubs, and on the network cards. First, the network cards were designed to have full duplex (both sending and receiving communications channels), and on the hubs, a solution was made to dynamically create a bridge between two hosts that wanted to communicate (based upon the incoming Ethernet packets), to switch two nodes onto the same full duplex circuit, much like the telephone system, but unlike the telephone system, this bridge only exists for small periods of time, and is wiped away when it is no longer needed.
This solves the collision problem because the two sides are only listening for the other node, and can negotiate when to talk, and with full duplex communication, they can send and receive at the same time.
However, this can't be employed with current wireless techniques for various reasons. WiFi still is very much classical Ethernet like transmission, except over high frequency radio (using spread spectrum techniques to cut through the traditionally noisy 2.4GHz band, this is part of the reason.) So WiFi clients all talk talk talk, and collide with each other quite frequently (most Wireless Access points basically no longer put the collision lights on the box, they would tend to stay glowing on busy networks, freaking people out.)
Long story short, WiFi is okay for some uses, but for uses that require a reliable connection (such as video streaming to multiple points, or reading data off a wirelessly connected network attached storage), you will start to experience performance degredation the more devices talk wirelessly. This can be mitigated somewhat in larger installations with multiple access points connected through a bridging mechanism like WDS, but the problem is one of current generation WiFi, and will be there for some time until a reliable way to create a virtual switch between two wireless MAC addresses on the same layer 2 access point becomes possible.