I hate it when i leave mine at home and pick it up at the earliest opportunity. I have four children, three of them teenagers, who are generally out and about and have their own mobiles. It would worry me if they couldn't get hold of me if they needed to. My number is also used by my Homecare clients, who have rung before to say they've fallen, can't get up; can i get milk on way over, etc. Also, i work with my sister, who may need to get hold of me/ need her children picking up from school or something.
I do feel uncomfortable without it, but looking at what I've just written, is more for other people's benefit than my own.