Traditionally it really depended on what was being done. For sure in Kenjustu uke was more often than not the senior but again it depended on what was being taught. In jujutsu training (which includes aikido) you often spent alot of time receiving the technique before you started performing it yourself.
In my opinion, part of the problem with our training model, was the change away from the uke being the teacher (this was/is the predominant model in koryu- from which modern budo evolved from). When the uke is in the role as the teacher, that person is in the perfect position for guiding the nage in improving the execution of techniques. The teacher can increase resistance, change things up, etc. as part of a training paradigm. Without that awareness, the uke frequently acts in a manner that is essentially nonsensical when they are acting against the execution of a technique.
As a teacher, I frequently intervene when a student is acting "dumb" in the role of uke. For example, last night, one relatively new student (almost one year of study) was resisting the execution of a technique in a manner that was not smart. After a verbal explanation as to why he was not acting like an intelligent attacker, he soon resorted back to what he was doing. I had him attack me and when he did the same thing, he suddenly felt an atemi in the ribs which was integrated into the execution of the technique. Frequently, the body learns before the mind
. I spend a lot of time teaching my students how to be a good uke. That means making a good attack without stupid openings; receiving the technique while maintaining connection and structure; and taking ukemi in a safe manner.
Our training paradigm is essentially a two-person kata practice. When both people take their roles seriously, the level of training can always increase, without unnecessary risk of injury, or devolving into some cooperative, delusional space where everyone feels good....