[ams6110]

for whom is the usability measured

I think this is critical. A software product like Emacs would never have been produced as a consumer product for sale by a vendor. Emacs is the way it is because it was developed by and for the people who used it: coders. Not all "free" software is targeted at nor should it necessarily be usable by your mom (not a dig at your mom).

(From the article) Free software developers mostly develop software based on their own requirement and their definition of “good software”, and as a result, design software that is very complicated and “geeky”.

In many cases, this is as it should be, unless they are developing something specifically targeted at users from the "general public." However, where I do think some projects go off the rails is when they develop a UI that is either internally inconsistent, or so non-standard that even technically-minded users are frustrated by it.