My main concern is security. When Twitter stores your payment information it will be easy to buy something with a single click. But I've seen way too many Twitter hacking going on when I look at friends' timelines. If tweets like 'How I lost 100 lbs in 3 months! Click > bit.ly/youwillbehacked' can be posted without too much effort, these same hackers could buy stuff for free by just hacking into your Twitter account.