I wouldn't do this for one simple reason: it might become an outlet for your customer's emotions. An 'other' box stays within the rational spheres; as a user you're being asked for a reason, you're not being asked about how you feel about the reason for canceling.
When you ask people why they cancel a subscription it often has a negative reason: "too expensive", "doesn't do what I want to do with it". Giving predefined answers lets you stay within the sphere of rationalisation and arguments. It also helps you pinpoint why you are losing subscriptions.
Giving a 'Let me tell you' or 'Here's why' option would probably call upon a user's emotional state about the canceling too much. You'd get an emotional response which is much harder to process and might cement their decision to dislike your service. They'd be reliving the experience so to speak as they are writing their emotions down.