Yeah, it's a bit of an advantage for the last minute guest but with my "WDW transit" hat on, I'm not going to spend $100 million for them and instead will just have them board the next train. Also, am I imagining things or do the cast members not really allow boarding at the last minute? Mostly I'd be looking to optimize for MK closing anyway.I think it’s something that would be nice but not hugely necessary. One of the advantages I could see is when loading, a lot of people don’t like to walk all the way down to the ends of the platform and block the isles. This way people could pile into the middle and still work their way down. Could also help for loading more last minute guests since the middle would even out more you could jump on even if it’s crowded and know that people will move down.
I do wonder about the other case you mention about people not walking all the way down to the end. I don't know whether it's because of my years of experience optimizing my NYC subway ride, but I'm trained to take advantage of these cases. On the subway I will almost always walk to the end of the platform to get the highest probability of getting a seat or at least not have to stare at someone's armpit. On the monorail, I will walk down as far as I can in the "holding corrals" until the path is blocked, then when the corrals open I will take a sharp turn and head to the end.
Good observations, though! Can't wait to hear from others!