I think that works when the solicitor already exists as a relationship, but I think he may be correct in that if the solicitor is not already attached to the donor then it doesn't pre-populate.

Although there have been times I wanted to do this, perhaps an appeal linked to just one solicitor who was making personal asks, but there are many times when I don't think I would. Some of my appeals may have 20 solicitors assigned to them, I wouldn't want all of them assigned to any gift with that appeal.

Are you entering gifts via batch?  If so, have appeal field listed before solicitor and when you select the appeal if there is a solicitor linked to the appeal, their name will auto populate in solicitor field.  That's been my experience.