If you have seen as many parents try to circumvent the system with their kids so that they could ride Space Mountain you wouldn't think my statement was out of line. I have seen parents actually hold their kids up off the ground or on their tiptoes in order to get them on the ride. Even after the CM explains that the height requirement is there for the safety of the child, the parent will argue and threaten.
I'm a parent. I have had three boys and I know what its like to explain to them that they are not able to ride because they weren't tall enough.
As for the height requirement being all over the place, I don't know what criteria they use, but I'm sure that Disney is just being extra careful in this "sue-happy" society that we live in today.
When I first rode Space Mountain in 1975, I was 13 and my younger brother was 11. There was no lap bar. Just a seat belt that ran across your hips. At one point, my brother's seatbelt came unbuckled. If he hadn't been sitting directly between my knees and I was able to hold onto him, he would have flown out of the car during the ride. So I can completely understand why someone too small should not be allowed to ride. There can be real danger on some of the rides if the safety rules are not being followed. It just seems like some parents don't care about that. They just want their child to go on the ride. I just don't understand it and I see it EVERY day.