Desktop boot time mostly doesn't matter, it is an infrequent event -- once a day at most, if the system is shut down at night. Server boot time, on the other hand, does matter. You really don't want a server to be down for longer than it has to. Being able to reboot in 10 seconds is huge compared to rebooting in a couple minutes.

Now, given that servers are fundamentally running the same OS, improvements geared towards that market tend to bleed over to desktop as well. Desktop just has a bit more to bring up, with a graphical environment and everything.