-Not everyone wants to or has the skills to be a creator.
I'm not a photographer. My pictures are barely good enough for myspace photo albums. No amount of coercion is going to get me to take and upload photos to flikr. It's only going to make me not use the site at all.
Right up until they were sitting on a billion dollar valuation, you could not even make your photos private to just your friends. Sharing was mandatory.
Note that I am not saying that zero content should be available until the user has made a contribution to the site, just that the site should at least keep something special for people who do. Most importantly, I am not saying that this practice is best for all sites or even the majority.
I agree. On myspace, for example, much of the value is tied up in the actual user. Quality photos aren't important. What's important is that they are photos of a real person you can make comments on and send messages to. It makes sense for them to encourage participation to a greater degree.