If you are leaving bruises, black and blue marks on your partner during practice, you need to learn to use the pressure points of Aikido, which leave no bruises or marks. There is definitely the opportunity to give each other pain, but then again how often do we reach into this area without extended training in sensitivity, transition, and flow?
It is incorrect to block out the pain, as it leads to injury. To be able to resist the pain up to the point of feeling immenent injury ... that is the area I feel we should attend to in the sensitivity of Aikido practice.
Of course, gentle practice will not cause injury or pain, but sooner or later, you will want a more atheletic, or taxing practice that raises the bar in speed, motion, and interaction. What you gonna do when it gets going and you don't have the capacity to stay within safe parameters because you have not raised the bar of your practice enough times?
Both uke and nage need to train on both ends of the scale to make Aikido a safe practice.