Agreed. These prediction contests are easily some of the best contests in this forum primarily because they are free to enter and skill does help to some extent along with luck while predicting BTC's price.
Its popularity has encouraged other sites like Sinbad to run similar contests using similar rules.
I have also come to know about Sinbad prediction contest from your post and yes they have been inspired through Bitcasino prediction contest only but they have less participation I think although have not checked about the latest one.But yes it's fun and if lucky you can win mbtc with only 1x wagering requirements which is good condition according to me.