Just came back from Starbucks. Did some inquiry and here is what I got. After 31 Dec 2011, no more 10% discount from J Card, HSBC and another loyalty card that currently also offers 10% (could not remember what card already). You will be able to get 10% from Ambank card only. For points system, you will still be able to accumulate points with your Genting and BCard. 
Nop, you can't. Cause you will only be able to transfer funds between your own cards. And registering 2 cards under 1 email account only entitle you for 1 free drink and not 2. Thought of it and did some research but found out can't.

Edit: Ahhhh, I remembered. Borders card. No more 10% from 31st Dec onwards also

to my understanding, ambank 10% is only if the purchase is paid with the ambank card. Most likely you cannot get 10% paying with the starbucks card and just showing the ambank card.

not sure abt splitting the purchase between the two cards tho

If one can afford the drink, what's wrong with it? And it's Starbuck's business to do this kind of marketing to promote their drinks. One does not open a business and waiting for the fortune to drop from sky.

Obviously, this is Starbucks marketing gimmick, which nothing wrong because they are running business, not charity.

Never heard ppl bankrupt because drink Starbucks.

So, if you're afford and you like to drink Starbucks, no harm for you to get a loyalty card to get more advantage via spending.
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For health issue caused by caffeine, may be this is not the right corner to discuss. If you drink Kopi-o at Kopitiam, you will consume caffeine also. Or you shud try de-caf drink at Starbucks.