You ask a horse for intense exercise and when you finish you just drop them out. Is this how you like to work out? For intense weight training maybe yes. For fitness aerobic work, not so much. Having to come in and out of a routine can be quite annoying. You lose your flow and each time you start you have to find the motivation to re-engage with the work. Same for long breaks, the longer you break the more difficult it is to come back to work. Even for intense weights I normally only stop for a couple of minutes at most while I catch my breath and my muscles recover a bit. I think for aerobic work for horses it pays to go from fairly intense to less intense, but not to stop. And for hard work like piaffe only stop briefly and then maybe drop back to easier aerobic work. I used to be critical of letting horses walk during breaks but when I think about it when I take breaks at the gym, unless I am super tired I normally keep walking and moving as it fits with the energy in my body and keeps me motivated for the next burst of work. Why should your horse be any different? Why make your horse stand still and have to do mental work as well as physical work?