like another poster said. . .
parenting is not a simple thing
I think every parent should use their best judgement when it comes to the rides, if the kid is mature enough, tall enough, and the parent goes on it with him/her. . .then why not?
Within reason. I think there are some parents who make us other parents of "smaller" children look bad by either a) taking a four year old on a ride as intense as Mission Space regardless of being tall enough and b) making their child go on a ride they really don't want to.
Now, there is a BIG difference between forcing your 3-4 year old onto a ride they don't want to and "forcing" (I think persuading would be more of an appropriate term) a child older than that, especially when you REALLY know they would like the ride or they are old enough that the term "need to try everything at least once" really fits.
Boy, I'm facing the "should I let him" dilema right now too.
My 4 year old is going to be tall enough (barely) this time. I am for sure taking him on Splash Mountain. But he really wants to go on BTMR and Tower of Terror this time. (He wants Test Track too, but it'll be closed

) I think I might let him go on BTMR, but Tower of Terror has me torn. I LOVE that ride, but am still unsure of whether or not to let him go.
Not only do I have to consider him and his experience. . . I am also afraid of what other people will think or say if I go ahead and let him go and then he HATES it and cries. . . . they'll probably all assume I made my poor son go on this big scary ride. . . they won't know that after watching countless Disney planning videos and travel channel programs my son has been begging and pleading to go on it.
Like I said. Parenting is complicated.