I've got Your Number

If I've seen it once (1), I've seen it one hundred (100) times: lawyers spelling out numbers then following them with the Arabic digits in parentheses. Why would anyone do that? I've heard some suggest it makes things clearer. My guess is the only reason they do that, is they've always done it that way. I want to know who's really going to confuse "one hundred" or "100" if either one is written by itself? As a general rule, a contract will be enforced as long as the intent is clear, and what's more clear than just saying "100." On the other hand, writing something twice is tempting fate. I'll bet people make mistakes doing that about two percent (1%) of the time.