In Sean's prolific interview on VentureHacks (http://venturehacks.com/articles/qa-sean-ellis), he recommends not charging before P/M Fit because you allow people to explore all of the functionality and can continuously survey them until some are saying they would be "very disappointed" without the product. I'd like to see this idea discussed further.
Playing Devil's Advocate, Charging is Good Because
1. Creates less noise, you can listen to the feedback of people who are paying
2. Proves people will pay for your product
3. Doesn't de-value your product