I think this is a very hard topic and no doubt I am going to expose the flaws in my thinking here. If I may list the advantages and disadvantages as I see them:
AIKIDO WITH A STRONG/STATIONARY UKE
Uke develops strong forearms (personally, I believe strength is important, if you disagree think about the experience of receiving nikkyo from your sensei)
Nage has much better control over uke ie uke can't chose not to blend because he is completely controlled
Students at a brown belt level are clearly capable of defending themselves
Students have precise technique with powerful movements
Quite stilted movements, decreased range of techniques
Why do aikido if you are not going to be free flowing?
AIKIDO WITH COOPERATIVE UKE (sorry colleen)
Developing sophisticated flowing movements from an early stage
Emphasis on creativity
Nage can get away with sloppy technique for years
Takes many years to develop self defense skills, if ever, in some cases - remember even the most famous soft sensei (eg Tohei) started out as very hard
Often breeches basic principles of self defense - turning back, awareness etc
In summary I think if you practice against resistance then you will have much better martial skills, but that is not what aikido is about and you may be missing the essence of the thing (if anyone finds it please tell me).