Ok, I see your point now. I agree that the heads should be kept for rebuilding, however they don't need to be. Price doesn't make something disposable or not. There are many people out there that would pay the price for new heads rather then fuss with rebuilding. Just because it can be rebuilt doesn't mean you must rebuild it.
I don't use cartos personally, so I don't know how long they last, but I thought it was just a few days. My vivi heads have lasted 3 weeks so far. I would think that longevity is the goal (aside from the obvious taste and vapor production).