First, whether conscious or not, too many staff have low expectations of the skills or qualifications of potential volunteers, and so design work posing minimal risk if not done properly. Both of these approaches are seriously flawed. Worse, they lead almost inevitably to conflict. So self-fulfilling prophecy runs its course. If someone with higher skills winds up in these assignments, she or he will soon chafe to do something more challenging.
Volunteer work becomes totally associated with employee work.
I'd like to offer a suggestion. The object is to try to wipe the slate clean and see what would happen if you started from scratch. You can see how these questions open up new possibilities. I'm not saying it's wrong to assign volunteers to help employees. It's just limiting.