I think they are a very good idea for people starting out in the hobby. It lets you make a better beer with little more effort than a basic kit.
I also think it is likely to give the more reluctant experimenter a gentle push in the direction of designing their own recipes.
In my own brewing, I tend to buy ingredients with a beer in mind, but I always design my own recipes, as that is part of the fun.
This approach results in leftovers, which results in Ready, Steady, Brew! beers based on whatever I have left over. Some of my favourite beers have been made this way.