I usually shop for other people - but it doesn't bother me in the slightest. Of course, I get what's on my list first unless they are needing something that is liable to go fast, and then I'm usually picking up 2 - one for me, and one for someone else.
My niece used to shop BF with me, even though she didn't have kids she just went to help me. Well this year she has a newborn, and her dh always works BF, so I'll be taking care of HER this year.
My mil, well, she usually only wants a couple of things, but really - she'd get hurt on BF, and she stays the night at my house on Thanksgiving so my dh can go shopping with me and we don't have to wake up the kids - so of course I'll shop for her. She's my absolute favorite person on BF morning!
A girlfriend of mine usually has me pick up one or two things - but then, we swap. We go to different stores and pick up what we can for each other.
For me, BF is all about teamwork - I'll even help complete strangers get what they want if I can. Since I'm usually at the front of the line, I generally can't really get past the throng anyways, so rather than fight the crowd, I help them so I have an easier path out.
I've never had any problems getting everything I want on BF while still helping other people.