if there is no debate, but veto, then this is not democracy , but dictatorship(turn on c-span i garauntee that congress isnt being graced with the presence of the president) congress debates among themselves, not with the president, if a president doenst like a bill, he can veto it without talking to congress.
for congress, debating a law is important to make proper amendment to meet the standard and to make the language of law tight, through debating, problems are exposed and corrected. yes they are often corrected, but they are often misinterpreted and really made worse.
In addition, only through debating, one is able to state out all of his or her opinions. otherwise, it is same with a majority-ruled dictatorship