First of all, suspension of the writ means they are not charged. But suppose that the next time Congress is out of session, thousands of people come storming out of mosques after Friday prayers screaming death to America and then suddenly bridges along 95 and 270 start blowing up, the airport, the Capital building and the Pentagon start taking random mortor rounds, and suicide bombers are blowing up on crowded metro platforms.
We kind of know the source of the problems, but we don't know which individuals may be the problem. We have a very nice profile of likely suspects, but we have no direct evidence on any of them.
You're the President. What would you do?
Arrest a lot of people on a charge like criminal conspiracy, and wait to see how the courts handle the cases. I think the people that wrote the Constitution made the judgment that that would be preferable to holding people without any charges specified. And I think they were right in that judgment.