Here's a little secret that law admissions officers don't want you to know. If you go up to them at a law school forum and ask, "Should I write an addendum about [insert blemish on record here]?" they will almost always say, "Yes, that's a fabulous idea, I really want to hear about that sad tale in your application," even if they complain bitterly to themselves about all the many stupid, whiny, zero-value-adding addenda they receive year after year after year.
Why? Because saying "yes" gets you out of their hair much faster, and it gets you out of their hair faster because that's the answer you wanted to hear. Because saying "yes" means they get to watch you walk away happy and eager to apply and full of warm and fuzzy feelings about how kind and sympathetic those people at School X are.
If they say "no," they'll have to spend all this time deflecting your follow-up questions, and tapdancing around the fact that your reasons for flubbing your first LSAT are pretty dumb, or that there's really little you can say in an addendum to mitigate a GPA that is way too low for them. It's much, much easier to say "yes" and then ding you later when they're not looking you in the eye.*
Reason number two: Remember that they want to drum up as many applications as possible and ding as many as possible to help their rankings, so scaring you off with a "no" would not be in their self-interest. They want you to think that you can talk your way out of a deal-breaking blemish with a simple addendum. But think of it this way: How irritating would it be to read gazillions of essays from people about how they really do look like Angelina Jolie, that the stupid mirror is "undermeasuring" them? That's what a lot of applicants are effectively doing in their addenda, and you bet it gets irritating to read those over and over again.
Bottom line: Most addenda I've seen do more harm than good. I hope this gives some peace of mind to the applicants who drive themselves into a tizzy over the conflicting advice they're getting about addenda, for example on this recent discussion board thread.
* Keep in mind that there are things you can do to mitigate a bumpy transcript, for example taking more courses to show them a cleaner, more recent set of grades, but doing is very different from talking, and addenda are usually all talk. And as I've written about in more detail in my book, I've rarely ever seen an effective LSAT addendum — didn't prepare for the first one? Forgot to cancel? Decided not to cancel and took your chances after some guy's cell phone went off during the entire reading comp section? None of those reasons reflects well on you in an addendum; just let the higher score speak for itself. Arguing in favor of the higher score is much less important now anyway since the ABA stopped requiring schools to average scores back in June 2006. Schools have a huge incentive to take your highest score, regardless of what they say about taking a "holistic" view of all your scores.