Optimally, headphones would have lemo connectors or something, and you could change the length to suit your needs -- for example, there are times when I am in front of the computer, and don't want to move -- in which case 6 feet is fine -- then there are times when I want to listen in bed, in which case 20' would be more appropriate, and so on.
Because we all know that extensions add additional connections, which are bad, right?