Keep in mind that even after these units are no longer "profitable" they will still be generating coins at a lower cost than buying them outright for some time after that. Many people would consider this to be an added benefit, as they will generate "x" amount of coins profitably, and then "x" amount of coins at a cost lower than outright purchase for some amount of time. If you factor this in they have a longer lifespan and I believe they would likely save enough money to be worth it in the long run for someone that is just trying to generate coins over time.

I haven't actually done the calculations, but that is something I consider when I argue with myself over whether I should invest in expensive gear.