An uke that holds relaxed is like my son, daughter or wife holding my hand. There is no intent, and with no intent there is no need to protect yourself. There must be action to cause reaction. A parked car poses no danger, yet a moving one does.
I agree. One of the things overall, that seems to get neglected in most Aikido training is the role of the Uke. Specifically, not only staying connected and "in the moment" with Nage so that reversals are possible, but also "Martial Intent" or in a way "Malicious intent" in an acting sort of way. Many Aikidoka, especially if that is the only art that they have ever known--in my experience--have a more difficult time playing the role of a good attacker- who means to do harm (safely of course--but true to form). The attacker by their very nature is by definition not coming to the interaction from the same frame of reference as the Nage. To not keep that in mind, in my opinion does a disservice to Nage's training.
Moreover, I always tell my fellow Aikidoka, especially new students and the class when I am privileged to have a captive audience when I teach, is to explore the role of Uke a fully a possible ( that may mean being open to breakfalls if appropriate of if Nage wants to work on them, having vigor in ones attack, looking for a second opportunity to attack during the interaction to avoid the "one and done Aikido mindset, etc) in every way, every time one is on the mat. Because father time will rob you of you ability to take the type of ukemi that you want to take or is required long before he touches your waza. The sands keep running though the hourglass.