I think the word churner applies to those who apply for credit cards, meet the min. spend to earn the signup bonus, cancel the cards (if they have an annual fee) and repeat the process. Each card has it's own limitations on how often (if any at all) you can sign up for the card again after cancelling. Citi seems to be the most lenient.

What's the reason for closing the MP card? What is your credit score, total credit and credit history with that card and your other cards? All those play a factor and you don't want to mess up your credit score by making the wrong moves. If you had the MP card for a while and it doesn't have an annual fee, I would just keep it open unless Chase forces you to close the card to open up another one. I had 4 cards with Chase and recently tried applying for a 5th. Was told to close one to get the newer one. Even though I had good credit with them, I applied for two cards just a few months ago.

Depending on your total credit, debt to credit (how much you still owe to your current credit cards) ratio, credit history, etc. closing a card for no reason will probably hurt you more. If it's a more recent card and everything else is good (low credit utilization ratio, long credit history for other cards), you should just see a small drop in your credit score. The more credit you have the better it looks. So by closing your card, your credit limit will drop.

I suggest reading up on these threads in FT and other blogs. They have a ton of information on churning cards. I wouldn't just jump into it without being somewhat prepared on what to expect and how it impacts your credit.