A single size 10-12 ESP hair rig costs £2.50 in my local tackle shop, so if I wanted ten I'd be spending £25.
To buy ten hooks, ten swivels and a roll of hooklink material costs about a tenner, and I've got the satisfaction of knowing that the rig that caught the fish was made by myself.
I do have a couple of ESP ones, but I by make my own now.
Horses for courses, if you have the skill, patience and confidence to tie your own, it's great, if not, then the pre-tied ones are fine for general applications.