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It seems that they're letting members who regularly contribute original videos share in the advertising revenue these generate.
It also seems like members retain the freedom to upload / distribute videos elsewhere - does this effectively mean that the creator of the video retains ownership of the copyright?
So, if you are already contributing original content, why wouldn't you want to sign up? It looks like people in this position can only gain by becoming a partner (unless I'm missing some important piece of the picture)!
For hobby type video producers this may be a good way to generate revenue by doign "their thing".