Some here will probably say you don't... but I think that has changed for at least some people. Today, WAN speeds of 50Mbps are pretty common, and 100Mbps is not uncommon. But even assuming 50Mbps, you might say that a 100Mbps router is fine. Lets think about that.

Packets are not 100% efficient. TCP/IP is the most common protocol used today, and efficiency is a function of how large the packet is. Efficiency varies from a low of about 50%, to a high of about 97.5%. So, if a large number of packets are small, the router will be right at the limit of keeping the WAN pipe filled. This is however unrealistic, and in reality averages of 75% are pretty normal.

If you doubt the above, here's a nice overview from someone who knows the info well: [Link: tamos.net]

Another thing to keep in mind is that often users will want to transfer files between computers, and will also do things like attached DVR's and other things which will consume network bandwidth. As you start adding these other devices, you can easily get to the point where having a local switch of 100Mbps with a single uplink to the router would be a problem. A single streaming 1080p assuming H.264 is 5Mbps, and H.264 requires quite a bit of compute power for decompression. So assume several of these streams, along with a houseful of users serving webpages on mobile devices, along with a file transfer, and the overhead of all the above, and 100Mbps is getting close to full, if not filled.

Gigabit networking is pretty inexpensive, so sidestepping these issues makes managing the network incredibly simple, and provides lots of bandwidth for file transfers over the network, streaming video, and access to the WAN at the full bandwidth of the WAN connection. At today's price point for Gigabit routers, the question is more aptly put, why wouldn't you use Gigabit?