It depends on who you have in front of you and what you mean as resistance. I give you an example.
Let's take an iriminage as example. I attack Nage and he enters behind me. From there if Nage is an experienced Aikidoka would make no sense in me putting my head on his shoulder, go round and round and the lie down for him. He would learn nothing from it, or better he wouldn't improve his Aikido.
When I do Uke for Sensei or Senpai (very experienced) I do put resistance in my role...of course if I can since they are so good compared to me that it is hard to actually "get them". But they have actually to move my head and have me walk, otherwise I stay there (well more than stay there, turn and attack them). When they start moving me around I try to run away, not that I ever manage to. When they place the other arm in front of me I use my hand to try and block their arms giving them as much resistence as I can. As I said, not much success because they are way too experienced for me to give them what I would like to.
When the technique arrives to the end and I lost all my balance and I feel I am going down, well then I take Ukemi...no use in resisting there (even tho Sensei says I wait too long and I wait for the pain to come before taking Ukemi...and maybe he is right...gotta work on that).
When I work on another guy who is more or less the same level as me I start easy. When I see that he can manage rather well the technique, I start "creating" him some problems. If it is too much, he himslef will ask me to tone it down a little bit...and the same works for him when he is my Uke.
Apart from the fact that if Uke gives you something to work on puts in the practice alot of fun, it is useful too. We all like a Uke that is 100% Uke, but he has no use for our purposes, which is to learn. If Uke tries to block you the technique will no longer go smooth and that is when you tend to start becoming "the hulk" and using all your muscles. An Uke who opposes resistance is the Uke which will force you not only to learn a technique, but to learn to actualle blend with Uke and adjust your position/movements to let the technique flow smooth and not use muscles.
If Iriminage is called the "20 year technique", I think Uke should be called "the lifetime technique". Becoming a good Uke, help perfectly your Nage (not too easy, but not too hard so that Nage won't learn anything), avoiding to get hurt, keeping your center and feeling the whole action to be able to counter...and why not, make very graceful and "good looking" Ukemi is as much a hard task as being a good Nage (if not harder).