During his August 9, 2013 press conference, President Obama discusses the difference between basing opinions on evidence of abuse versus prospects for abuse. He doesn't seem to think that the latter is as good as the former. (He also doesn't seem to be aware that there is evidence that these programs have been abused.)
"And if you look at the reports — even the disclosures that Mr. Snowden has put forward — all the stories that have been written, what you're not reading about is the government actually abusing these programs and listening in on people's phone calls or inappropriately reading people's emails. What you're hearing about is the prospect that these could be abused. Now, part of the reason they're not abused is because these checks are in place, and those abuses would be against the law and would be against the orders of the FISC."