I think there's a timing issue. Calling you five minutes before guests arrive for a ride is an unreasonable request.
Letting you know in the RSVP that there is a transportation issue and asking if there's anything you are willing to do to help is not unreasonable. It's communication. I'd rather have a potential guest tell me a specific reason for not coming...especially if it's a problem I can help solve, then just get turned down outright. I have had no problem in the past, sending my DH to pick someone up...or even asking a mutual friend who lives on the way if they mind picking somoene up.
I also think that calling a host the day of the party to let them know an unexpected transportation issue has come up is just good manners. As a host, I still might have some resources at my disposal to help them out. And while it might be an unexpected issue or a 'kink' in my plans, I really don't chalk it up to much more than "that's life".
There are unreasonable, entitled people in this world. But I don't think that just because some people out there are poor planners, bad friends, SS, whatever, means that it's rude for all people to have legitimate problems that they couldn't avoid and ask for help. If the problem is that you have a hard time saying 'no', then you need to work on that problem. And if you have 'friends' that are consistently in need and seem too expectant, then stop inviting them to parties. As long as the requester can accept 'no' for an answer, then I don't see a problem with asking the question.