Think of the warm up excercises as solo training.
Tenkan, tenkan, tenkan.
sorry to edit your post but the above is what my sensei says too. the warm up exercises at home are great, I find they make me feel much better after doing them if I can't be bothered going for a run or cycle. after a day in front of a pc, you need to loosen your muscles and stretch. anything that gets the blood circulating is good in my opinion.
a bit of ki breathing works wonders for relaxing the mind and body too.