The advantage is that you get a little softer touch and a little more bounce out from the hard sponged Chinese rubbers with their harder, tacky top sheet. Most of us don't want that "super bounce" that most of the tensor rubbers today have, because you lose the control that you have with Chinese rubbers. And the effect you get from brushing the ball with a Chinese rubber like DHS H3, 729 Battle II, Sanwei Target National etc. can't be reproduced with a tensor or a hybrid rubber like Tenergy, Dignics 09C, Rakza Z etc.
About boosting. I had a chat with a representative from the ITTF, and it feels like a big grey zone to me. Well. It's actually really black and white from the ITTF's point of view. Boosting is not allowed, they say. And any cleaning of the rubber that affects the performance of the rubber is not allowed (like Falco Life Expander etc.)
As it's only the top sheets that are approved by ITTF you may put any sponge on an OX rubber and play with it, as long as it doesn't alter the rubbers characteristics. But you may not boost the sponge before gluing it to the top sheet (but you may use any sponge you want) 
I also asked if gluing is allowed, as the glue works as a treatment for the sponge, and it pours down into the pores. This works differently as there are different viscosities in different glues, so I thought this would be relevant as well, but I never got a good answer in this question.