Paying extra into MPF (into TVC account) can be good IF your yearly net taxable income (income after basic allowance, married person allowance, child allowances, etc) is > $200k. This is because your paying tax on this @ 17%, but whatever you put into the TVC you can claim as a tax deduction up to I think $60k/year.
So lets say you put $60k/year into TVC MPF. That money is invested like normal MPF and hopefully grows. But you get a double benefit of a $10k reduction in your tax payment each year (17% of $60k). So this is why even if MPF isnt the greatest investment in the world, it can still be a good thing.
If you are a local or planning to spend the rest of your life in HK, you can't touch this TVC until you are 65. So this is the downside to putting extra money into your MPF TVC. But if you are expat, you can just rock up to the local government office and hold up your hand and say "I declare I am leaving HK permanently" and then withdraw it and get it all back. But you can only do this once in your lifetime.
So MPF TVC is usually more attractive for expats than it is for locals. I will go as far as saying if you are an expat and not planning to live in HK for the rest of your life, you really should be putting $60k/year into your TVC otherwise you are just paying an extra $10k/year in tax for nothing.
Also if you are leaving HK, open the TVC account and dump $60k in it, rock up to tax office and get $10k wiped off your tax bill, close the TVC account next day and fly out of HK. Its a $10k farewell gift for any expat leaving HK permanently.