Diversification is key. It's better or matching FD rates. Unless you believe you should put all your eggs in the high risk high reward basket. Of course it's also stupid to put all your funds in SSPN, but this is only 6k a year.
Can, but the amount you put back must be 6k more than what the amount was on Jan 1st.

So if you do the following:-

Dec 31 2012 - Deposit 6k (qualify for tax deduction 2012)
Jan 1 2013 - withdraw 6k (takes a few weeks though)
Dec 31 2013 - Deposit 12k (qualify 6k for tax deduction 2013)
Jan 1 2014 - withdraw 12k
Dec 31 2014 - Deposit 18k ..... etc.

Then you get the tax rebate, and the only loss to you is the lost interest of a few weeks of processing for withdrawal.