I think that kits can be a rip-off for people who know what to buy and where to get it. I would be reluctant to buy them, except for patterns that require numerous balls of different coloured yarn that will only use a small amount of each, and cost a fortune to buy a whole ball in each colour. Even then, because you are only getting the quantity of yarn specified in the pattern there is no scope to add length, or otherwise alter the pattern to your taste.
The ones you describe are usually aimed at beginners, and use chunky yarn and large needles, and cost a lot for what they are. I've seen very basic baby cardigans for £70, which seems insane for a chunky but tiny garment that will only fit for a few weeks and you have to knit yourself!
I suppose they make good presents for someone who wants to start knitting and has no needles or yarn 'in stock', but otherwise they represent a large outlay for someone who doesn't know if they are going to enjoy knitting.