Okay, this totally happened to me. In real life. Twice.
Woe Pt. I: You're So Not My Reader - I had this friend who I was really close to in high school. She was a year younger than me, so it was going to be hard to stay in touch. BUT, she ended up getting engaged to a boy who had lived down the street from me from age 2 to age 14. He was a great guy. She asked me to be the reader at her wedding, which was going to be two years after I graduated high school. We talked occasionally, and I saw her when I came home for holiday breaks. Eventually, she stopped calling/writing me (this was 1994-1995... no one really had email in my Podunk town. It's true! It's not just a story parents tell their kids to scare them!). Suddenly, it was like 2 months before the wedding and I got mono. And was so sick. And thought I was dying. I never heard from her again. Somehow, I got demoted from being a reader to not even receiving an invitation.
Woe Pt. II: Bridesmaid, Schmidesmaid - I had really hit it off with this girl I'd met at college. We were "twin sisters" in my sorority (it was a dark time in my life...) and were on the college flag/rifle team together. We were both dating drummers. It was like kismet!! One of my BFFs and I were supposed to be bridesmaids in her wedding. Well, she graduated a year early and was only sporadically in touch with us. We both left her messages asking her what we needed to do (especially after my previous experience), but to no avail. She just stopped calling. This time, I was sure it wasn't me! I mean really... Could it be me? I guess it was a beautiful wedding. I wouldn't know, because again, I was somehow left off the guest list. WTH?
Miss Manners? Right. Here's what led me to tell you my tale of woe...
Dear Miss Manners,
What's the politest way to kick someone out of your wedding party who has repeatedly not held up her end of the "deal'?
What was the deal?
If it was that your friend must accept the honor of waiting on you hand and foot while surrendering control over her own time and money and wardrobe, it was an illegitimate deal. Those terms exist only in the minds of brides who have worked themselves into a state of self-inflation that has rendered them thoughtless toward their own closest friends. Miss Manners is not going to help you fire a friend for refusing to turn into an indentured servant.
However, if your bridesmaid has been telling you that you can do better than marrying the gentleman in question and that she hopes to make it to the wedding but can't promise, it is she who has broken the deal. In that case, you could say, "I don't think you realized what you were getting into when you kindly took this on, and I can see it's becoming a burden to you. You know how dear you are to me, and I'd be just as happy to have you at the wedding without your having to go through all the business of being a bridesmaid."
We didn't have a wedding/bridal party; so wow, this was informative. In fact, I think we've all learned something today. I just didn't realize that some brides thought being a bridesmaid is supposed to be all about being a doormat. When you're not, you're so cast-off, replaced, and forgotten. I thought I had had a couple unique experiences. If this happens to you, hopefully it will be before you invest in taffeta.