I would venture to guess most users don't want to have to change their dimensions via the props box. I know I don't.
Most probably don't. It depends on the application I suppose. I think it makes perfect sense in some situations.
But in this case he doesn't want any grips at all, so the user doesn't know they can edit it. This accomplishes that. Plus most people neglect to realize that all their dynamic properties show in in the properties palette which should hopefully give you more reason to name all of your properties (and hopefully actions as well).
A great example of where the lookup is super helpful is say 10, 20, 30, 40 area all potentially valid heights but 30 is only a valid height when it is 20 wide. A lookup is absolutely vital there otherwise someone might pick invalid choices.