Personally, I know I couldn't do better. Otherwise, I would (what with PDF publishing and all). What FF is being compared to is a) the competition and b) reality. And people actually running or using their products also should be able to express an opinion of them.
This is an old argument that tries to demonize critics, by saying if you can't do it yourself, you're not allowed to comment on it. It's used quite a bit. But I've always thought it was the people who the product that it's aimed at that are the best critics. They might not be able to tell you how to make something better, only that it needs to be improved.
For instance, take Keanu Reeves. I would wager that many people who have seen his movies have criticized his acting. Yet how many could act better, or tell him how to improve? Very few. But most people still know they want better acting (unless it's a movie where he's supposed to look confused...)