Tracking HP manually on paper is way faster and easier than continuously updating the values using keyboard/mouse.
I would disagree depending on how you do it. Right now I use an MS Excel sheet to track combat (which looks a lot like your program). I don't "update" the HP values for creatures, all I do is enter how much damage they took that round and it automatically deducts it from their total HP. Also then highlights the creature's name in bold red when it gets bloodied.
As a fellow programmer, I know that you often want to show people the most effective way to do something, but I know that having to keep track of HP on a separate sheet of paper and then refer to this program would be a deal breaker to me.