Brian, I agree that in the beginning stages of training the attack must fit the uke's ability to deal with it. As the skill improves so does the level of speed and power.
However, there are lots of aikidoka that I've seen that have very little to no real intent behind their attacks. Often, attacks are delivered with the intent that an aikido technique needs a "handle" to grab onto, etc.
There's a difference between delivering a committed attack with real intent that is slow and has reduced power for beginners and one of these "fake" attacks. The intent must be there for the proper lessons to be learned.