I have the PTH module from a set of E500PTH and used it with my E3s. I don't bother carrying it around with me anymore. It was interesting to play with it, but its not all that practical. By the time you want to listen to something external and fidget around to find the box and then slide the switch (no idea why the call it "Push To Hear" when its a slide switch, but that is another story) the moment has passed. Much easier to pull out one earphone and re-insert when done.
You can purchase the PTH module separately if you really want it and don't want the top-of-the-line phones.
I have the E500s and find the PTH module utterly useless. It has an amazing knack of picking up all the background noise on the plane without picking up voices.