The article does not take into account at all that you not only rent hardware, but pay for a service (which includes testing and deployment of firmware upgrades) which you would need to do yourself. If something does not work with your own device, you're on your own. Of course, the time you spend with upgrades needs to be factored in as well. If you use the same (ISP-provided) firmware, you can ask peers/other customers in case of problems.
Also, in Germany, Vodafone configures a separate Wifi connection for "guests" (other Vodafone customers). In return, wherever you go, you can use other devices. Depending on the coverage--which is not too shabby in my neck of the woods--this is actually not all bad. With your own device, this is not an option.