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.
A parked car with poor brakes on a hill poses a great deal of potential danger.
If there is no intent, there is no attack, and no need (or opportunity) for aikido practice.
But "intent" and "relaxation" are not opposites. A relaxed grab can still contain a great deal of intent, and in fact contains a great deal more potential danger than a strong, rigid one.