Detachable cables have their advantages, but for full-size (non-portable) headphones I actually prefer fixed cables. Why? Connectors are weak points in a connection. It's best to use as few as possible. They can wear out over time and could even cause connection problems.
For example, the connectors on my HD600 are slightly loose - I think I will solder the cables directly to the drivers to avoid accidentally unplugging them (which happened a few times already).
Higher quality plugs and sockets could partially solve these problems, but they would also raise the price of a headphone.
Additionally, plugs and sockets for detachable cables will usually make a headphone slightly bulkier.
However, headphones with fixed cables should be easy to open in case you actually have to replace/repair the cable. The K701 for example, is easy to open - you only need tweezers and a screwdriver (although I admit it is easy to scratch the cover). The cable is not glued, so it can be easily replaced. Grados on the other side are horrible, I hope I will never have to open my RS1. In this case a detachable cable would certainly be preferable.
A question to those who like to "upgrade" their cables: Assuming that you can improve sound quality with expensive cables (I do not believe this, however I do not want to start a discussion), why would you prefer a detachable cable? Even if you use the highest quality cable you can find, the signal would still have to pass through the connectors on the headphones, and then through the cables inside to the drivers (which are most certainly completely conventional cables and not pure silver or whatever).
If I would want to use a High-End cable, I would most certainly solder it directly to the driver to ensure there is no weak link in the chain.