I like training with beginners, as has been said, it makes you focus on the technique's form, and as long as they attack sincerely it gives you a chance to work with an unsuspecting uke. Even if they aren't very sincere, it's still good to work on how to get the technique to work regardless. Anyone can look good with a good uke, it takes much more to be smooth and not hurt a beginner uke.
Even working with advanced partners for the entire class/night isn't exactly what I would call an aerobic challenge... if I'm not tired by the end I can always do extra rolls, I don't judge a class by how sweaty I am at the end of it.
I like the fun of taking ukemi for a beginner, it can be harder (how in the world did they get there from here? and how do I fall from this???), and is a chance to practice teaching via one's ukemi.
In exchange, I would expect two things: that they in turn work as happily with those junior to them later as I do now, and that they work on improving their ukemi.