I totally agree with Kellzea in the first part of its post, but totally disagree with the second parts of it.
Free subscriptions would mean that everybody will have access to the market, so there would be a lot of people creating apps just for fun, submitting crappy useless apps like soundboards, hot pictures albums, mobile websites embedded, etc.
And the actual cost of 19$ is more than affordable for anyone, so we would not gain any new good developer making the subscription free.
Actually, I think that 19$ is too cheap to keep away inexperienced kids from submitting stupid apps...
(I provide assistance in the msdn italian wp developing forum and recently the questions posted has reached an embarrasing level)