...but boot time does matter
"most Linux users tend to just leave the system running indefinitely"
Is this true? Does no-one care about energy consumption? In these days of energy efficient light-bulbs, and energy markings for TV standby modes, leaving a PC on consuming 100W or so during the night seems kind of irresponsible. Frankly, even buying one that consumes 100W if you could do all you want on a fan-less job at less than 30W seems a bit off.
Its a very smart move on Ubuntu's part to focus on startup time - the days of power hungry machines running non stop when no-one is using them must be numbered. You also save a bit of money (not as much as 26 seconds of a person's time 230 times a year, but probably more than 10 seconds of their time).