Usability most certainly /is/ an opinion. Yes, you can have people in white coats in a lab stand over people while they try to use whatever new interface you came up with and you can time how long it took them to figure out what to do. That tells you approximately zero about how good your user interface is.
Why? Because a person has to learn how to use your interface exactly once. A person usually has to use your interface much more than once. So, you should optimize a user interface for speed, not intuitiveness. Once people have figured out how to use your interface, they won't have to do so again, and they'll want to use your interface to interact with the program as quickly as possible. This is why vi is, in my view, one of the most usable programs ever written.
"But wait!", you say. "Nontechnical users might never figure out how to use your program in the first place if it's not intuitive!" That's absolutely right, which is why so many nontechnical users use text editors other than vi. So, you have to trade off between intuitiveness and literal "usability" in your UI design.
What's the appropriate weight of each of these factors in the tradeoff? Well, uh ... it's an OPINION! My opinion is that I want the programs I use to have intuitiveness weighted at 0 and usability at 100%, because I'm smart enough to learn from and patient enough to read a manual when I encounter a new program, and I don't want the speed at which I am able to use that program to be negatively affected by those who lack my intelligence or patience. I do not suffer fools gladly.
An emphasis on usability as opposed to intuitiveness was part of what attracted me to Linux in the first place. I had experience with DOS and wondered, "Why does Linux use 'cp' instead of 'copy'? 'copy' is more intuitive!", and was then able to answer myself, "because 'cp' is fewer characters, of course!" Open source developers tend to write software that they wish to use; since they are intimately familiar with their own software, this promotes the development of usable and not intuitive interfaces. I like that.
However, I am exactly one user, and I have no more moral right to dictate the design of software I am (generally) not paying for than the fools I do not wish to suffer. Interface design is a judgment call that each individual project will have to make based on the type of user the project wishes to attract.
They should remember I'll probably give more informative bug reports :p