The problem with "sucking what they can" is actually quite simple: When a router or switch is saturated with traffic, packets are lost. When packet loss increases beyond a certain percentage on TCP/IP network, performance for everyone goes way down hill. It is much better to cap everyone's pipe at a lower (but still reasonable) data rate so that packet loss does not ocurr.

If all connections were TCP/IP, packet loss would not be that big a problem; since a lot of people are using streaming protocols that use UDP these days, though, routers can become saturated very easily.