That's all well and good, as long as you don't use your home network to transfer data between the various machines you have; 802.11g or n is so much faster than 3G that it simply wouldn't be worth it.
Anyway, who mucks around with settings or changes IP addresses when they want to wander off with their laptop? Where DHCP is the norm, it's just a case of a couple of clicks to select an alternate connection, and that's only if their machine doesn't automatically shoose an alternate connection type when the active one fails (eg Ubuntu). Not exactly back-breaking labour, is it?
To summarise: it's a lot of money and some extra hassle so that you can slow down your home network in order to solve a problem that doesn't really exist for the overwhelming majority of the population.
Why? The best most people could hope for is that they never notice a difference.