It does help in training if uke is giving honest attacks but I have been often told "uke is never wrong", I'm not sure where this quote comes from but I do buy it. In the real world the enemy doesn't follow our rules so when uke doesn't give an honest, clear attack then in response our technique should change to accomodate this change. Often I find that when we try to get uke to attack a certain way it is so that uke can protect themselves and not get hurt.
I've heard a story related to this...
two aikidoka are training shomenuchi ikkyo. One of the new students is being difficult and making their shomenuchi a straight arm so that their elbow is locked straight, making it difficult for the other student to perform ikkyo. Meanwhile the other student is performing a normal shomenuchi allowing the "bad" student to get the ikkyo every time. The "good" student tries and tries again to no avail while the "bad" student shows his ego by resisting the technique. The "good" student gets the attention of sensei X(not naming names) and asks what he/she are doing wrong that he/she can't perform this ikkyo. Sensei X asks to watch it, and observes a couple of times. Sensei X has instructed the class to leave the elbow bent while performing shomenuchi yet this "bad" student won't do it to try to show a flaw in the technique. Sensei X now asks the "bad" student to shomenuchi him so he can show the ikkyo. Once again the "bad" student tries to foil the ikkyo by making his shomenuchi with a straight arm but as the arm comes down Sensei X drills him in the ribs with a reverse punch. This awesome blow of course knocks down the bad student and breaks his ribs as you can imagine. Sensei X says something like "And that's why you drop the elbow on shomenuchi." And I believe that the "bad" student was hauled off the mat never to return. Now....when the person that told me this story asked Sensei X why he went to that extreme he replied that he knew that this "bad" student had a bad heart and was not suited for study there. I'm sure I butchered that story but that was the gist of it.