If you succeed in CTBing while practising for CTB, isn't that just a succes (and in that way, not a bad idea, unless you don't want to die at all, in which case it'd be weird to try out CTB methods)?
If someone is serious, they won't mess about practicing in my view.
Doing such methods without being sure is a bad idea.
Look how many accidental deaths there are from partial from people doing it to get high or part of sex.
If there is any risk of being prematurely discovered by others, this could be a concern. What if you are practicing, accidentally pass out, and then someone "saves" you after your brain and other organs have been irreversibly damaged from lack of oxygen, but you aren't actually dead yet? This is more likely to happen if the event doesn't take place at the appropriate scheduled time and place.
You also wouldn't have had a chance to leave a note, or the timing could also just be problematic in a number of other ways. You could end up traumatizing a family member unintentionally, for example.
There are certainly risks involved; but they are also specific ways of 'testing' setups that work by the person holding the other end of the rope, so if they pass out, the tension is released. Obviously this should be done safely where you won't fall and crack your head open when you go limp.
I can understand people wanting to do 'dress rehearsals,' though. People want to have as much assurance about the reliability of their technique as possible; of course, nothing is ever 100% reliable.
People are also skeptical about the whether the technique is indeed "painless," they don't want to feel like they're being strangulated, so they may be trying to gauge their level of physical discomfort before trying the method.
On the other hand, some people may unconsciously wish for their attempt to fail from the outset, maybe they purposely sabotage some detail before proceeding, in which case, their wish may be for something other than immediate death. That is another matter.