2. No drunken scenes.Sure, they do make for one of the best things about a wedding, but you may not realize that on your special day. At least your animal bridesmaid would (most likely) continue to stand soberly by your side.
3. Totally comforting in a bridezilla meltdown. I've been Maid of Honor at several weddings, and at a few of those, there were some not-so-great moments when the bride just freaked out and started yelling about things that were, well, let's say not really important at all. At least an animal probably won't experience a temporary utter loathing for their bridal loved one – they can just provide some fur (or feathers, or scales, etc.) to cry into. And that's often the best remedy.
4. Add excitement to any wedding ceremony. Not everyone likes to sit through a wedding ceremony, but come on – if there's a horse or dog or something involved, at least that makes things a bit different, and even a little unpredictable. Remember that penguin who peed on the bride's dress at a zoo wedding? I mean, I feel awful for the bride, but I bet her wedding will never be forgotten by anyone who attended.