I like resistance, to a point. Resistance allows me to find the weaknesses in my technique and work on them. If someone keeps poking me in the ribs, it's probably because I'm not moving correctly, or my posture is off, or there's too much tension in my technique.
I've been to dojo's where all I have to do is touch uke and they fall down, that doesn't teach me anything.
That being said, the person in the most danger of resistance is the one resisting. A skilled nage will change technique because that's what uke is asking for. An unskilled nage will try to force the technique and may injure uke. Uke should resist enough to help teach nage, if it turns into a test of strength, or a contest of who's more macho and can take the most pain, someone will get injured.
Really awesome point about correcting posture but I think maybe the resistance you're talking about in the beginning is more on extension (as in, not fall until you need to, i.e. not just fall at a touch), and not "resisting for the sake fighting back" which is the kind of resistance I'm talking about.
In the case with the visitor we had, nage was doing iriminage, uke was already bending backwards when he grabbed onto nage's throwing arm with the outside arm, and then started jabbing with the elbow with the other. Unless nage did the technique SUPER fast, anyone could have probably done that... only, both nage and uke were next to a wall with 4 more uke's advancing... I don't think it was the right or safe place to be "testing" in that capacity then. =/