A lot of people first join because they want to list and rate a specific show though. Who wants to join a new site to make a watchlist they can't even rate? Technically, by your rules, sure, they could go back and rate it all later - but who honestly has time for that when they could just go to another site? This is a great way to alienate new users and create a ton of dead accounts where people sign up and realise "oh, this is useless".
It would be better to have something like new accounts' ratings not being applied (to the general rating calculations) until they've added and rated a certain number of items to their watchlist, with the account being flagged for human review if its ratings are overwhelmingly negative.