JACK is for the pros, typically also desktop users. They need super fine-grained control over audio, they need it to be in realtime by default and always, the system is probably locked down so security isnt an issue, and also Audio is the most important process other than the kernel itself.
Pulse is the middle ground. Pulse is for the everyday user, especially laptops (headphones and power consumption). If there's a few milliseconds of lag, thats fine, but realtime is available if needed. They're gonna be plugging in HDMI cables, or speakers, or running skype and music, or headphones. Basically if you need anything OTHER than "This is my desktop, these are my speakers and I do 1 thing at 1 time" (that would be alsa), you could probably benefit from running Pulse. Maybe not benefit enough for YOU to bother setting it up, but there would be SOME benefit from Pulse.