Ensuring your app loads quickly over the network is definitely an optimization, but to call it a micro-optimization is wrong. Size on the wire for html5 apps (frequently loaded over mobile connections) is completely relevant.
"Oh no, our application can't be simultaneously downloaded by 3 million martians because our client is too big." I'm being facetious, of course, but I think the point applies. Don't build stuff with weird contingencies in mind. The average user (of an HTML5 app) will not have to worry about processing power, bandwidth, or client size. As long as it's not like 20mb or something stupid.
And, obviously, websites are accessed from all kinds of connections. Not everyone has a generous 3G plan, and even the US doesn't rank that well in average broadband speeds.