The problem is that it's very hard to look back and know whether or not it was warranted. There is no control group. We don't know what damage if any was prevented. Clinton had a commission that tried to look backwards and decided that there was little justification to think the people interned were a threat. But apparently the Congress at the time thought they were.
You raised some good questions. One of the reasons for Japanese-government-connected citizens of Japanese descent being interned, was that Japan came closer to attacking the U.S.A. on our own soil.
The Niihau Incident seriously freaked out the powers-that-be. A single Japanese airman from the Pearl Harbor attack landed his damaged aircraft on the remote Hawaiian island of Niihau. He had no trouble getting the only three Japanese descendants on the island to help him out. Whether it was indicative of overall Japanese-American sentiment, I don't know.
I strongly believe the internment was wrong, but I was not alive at the time, and can't speak for the people that were.
However, I do agree with Scalia; human nature has not changed that much. A modern Niihau Incident could spark a drive to get conservatives into the concentration camps...unless we fight back.