Being open means that someone technically proficient could then update it themselves. Gruber is arguing that most users aren't that proficient, so being open doesn't compensate well for a manufacturer, like Motorola, failing to update their phones themselves.
I get that. But Motorola being incompetent doesn't mean being open is not advantageous - which is what the article seems to imply. Would you blame C++ if someone uses C++ incorrectly?
Take for example CyanogenMod - the guys involved in the project don't get paid by any of the manufacturers, but they are able to build ROMs that can run on a host of devices while providing a consistent stock-Android user experience. How is that for being open.
I agree with you but I think you miss part of the argument. Yes there's a very small number of users who are that proficient but even those who are would have spend a monstrous amount of time to update the phone and make sure it works. Time built creating something that people will discard a year from now when they buy a new phone.