The rationale for us shifting to pay in advance is really just as simple as what Kevin said in his post earlier....it is aimed at "reducing confusion that arises relating to billing and invoicing, particularly when users change their plan or account type."
Once we are shifted over to advance billing, if someone wants to cancel, if they do the cancellation close to the end of the month then they will have used up the service they have already paid for and it all works out very neat and tidy.
However, if someone feels they have no choice but to cancel partway through the month, they can contact support and request a refund prorated for the unused time remaining in the month. Our terms of service that people consent to when they sign up don't obligate us to give refund for the month someone cancels in, but as a matter of practice, we try to accommodate people.