Therein lies the very premise of aikido.
If you are not being attacked (a half-hearted/non-committed attack can hardly be called an attack), then there is no need to respond. To respond (e.g. "atemi") in such a situation makes you the aggressor, which is hardly "self-defense" is it?
In any case, there are ways and means of dealing with a non-committed attack, but that is hardly realistic nor representative of a confrontational situation.
Resistant ukes OTOH are a different thing. IME, it takes some level of skill to feel the resistance and realign accordingly to find the angle and path of least resistance in order to take uke's balance.
However, when you're initially learning, it may be necessary to "go along" until such time each person is comfortable with coming in a little more committed and a little more centered. This provides nage with a little more challenge in terms of finding the correct body alignment and kokyu extension paths.
One of the reasons I don't like to use "resistance training" is because it connotates the idea that one needs to respond to resistance with force. When in actual fact, a simple change in body alignment and angle, and correct kokyu extension thru uke's kuzushi point is usually sufficient to effect the technique.
Therefore, the person that doesn't "go along" is either going to get hurt if they can't do the ukemi, or they're simply not going to get the full benefit of the art. Ukemi is the way to feel the techniques working, correctly and safely. Good ukemi leads to good technique.
FWIW, my $0.02