a household of 4 networked computers 4 kids could easily hit the limit within 1 hour. its limiting just a bit more than pirates, and if you think the limits were brought in as a anti pirating measure, then your very short sighted.
It's a combination of both infrastructure and piracy problems... Providers are covering themselves to some extent, there will be a shakeup I'm sure in the future.
Even with 4 PCs on a home network, it's no big deal.. I remember having a home network of that size with only 600k 1st gen DSL.. was fine for normal use.
The whole way the industry is going is the word 'unlimited' is to be taken with a pinch of salt.. but hey, you pay a 'limited' subscription to the service, it's unreasonable to think that buys you unlimited use. (and for many heavy users illegal use)