This morning I saw a girl wearing a short dress, opaque tights, and flat suede boots and the first thought that popped into my head was elf. A common mistake I have seen among interns and recent college graduates is substituting an inappropriate length dress with tights or even worse, wearing leggings. Business casual is not "bar sheke." I cannot imagine any professional circumstances where leggings are appropriate.
This girl wasn't all wrong. While leggings should be banned from your work wardrobe, opaque tights are certainly acceptable, but only for the purposes of keeping warm or adding color. If you think your dress is too short for work*, it probably is. And no opaque tights will make that more acceptable. *Emphasis on work, weekend attire is entirely different.