Also, the icing on the cake is that you get more Chinese girlfriends.

Can't argue with that and it's a fantastic motivation for any new guy arriving in China to learn the language

I have to say though, as someone who speaks good Chinese I generally try to avoid speaking it in important situations such as negotiating a contract with an employer who can also speak English. My logic is that any conversation held in English between me (a native English speaker) and a non-native speaker i hold the upper hand. Why should i give up this advantage by speaking in Chinese? My Chinese is good but not native, there will be lots of opportunities for the Chinese speaking boss to pull the wool over my eyes in terms of the language that he uses should we carry out the discussion in his language of choice.
Finally whatever you agree in whatever language, write it down, preferably in a contract. Most contracts are translated in both languages and any good contract will clearly stipulate that in the event of a dispute it is the interpretation of the English version of the contract that will be used to settle the dispute.