score:4
Assuming you meant by China, is mainland China... I guess you better not throw it away. If some picky immigration officer will go scrutinize your travel history, you would have something to show for the period you spent on Hong Kong. If you don't have it that could be a red flag for them and I highly doubt they have immediate access to the database system of the Hong Kong Immigration Department.
BTW, the use of slip is not because of PRC-HKG relations but to save time. Source: http://www.scmp.com/news/hong-kong/article/1146055/immigration-department-end-visitor-passport-stamps . Apparently, you can ask the IO to stamp if you like. I wish I knew this so more stamps on my passport yay!