The argument about not being able to run SSH on the server seemed a bit weak, surely just stick two NICs in there, one for management + one for user-space stuff?
I think that's less an argument, and more of an example. You are correct, that is certainly something one can do. But I think the related argument is that now your system configurations are more complicated; you have tied your hardware and software together, for example. For some that may not be possible, and even if it is, some may not want to give up the abstraction that the kernel provides.