Declaring a state of emergency, while at first it may seem like a panicked move by a wounded nation, is actually a smart way to encourage caution and alertness in citizens. It also lets people know that the government is fully aware of the problem and is taking steps to fix it.
France is right to declare a national state of emergency. During this time they can actively search more thoroughly and capture outlying terrorists and their associates. In this case guilt by association is a valid verdict in the efforts to prevent further loss of lives. National security and national preservation of life are at the top of the priority list where they belong.
Declaring a state of emergency is not that effective in combating terrorism. Yes, the state of emergency can give security officials more powers to fight terrorism. However, the war against terrorism will have to be won on the battlefield by defeating violent groups like ISIS. Therefore, if countries are serious about combating terrorism, they must be willing to send the military forces needed to defeat it.
At best, an official state of emergency tells terrorist that terrorism is not going to be taken lightly. It can also backfire, because it will cause the people to be afraid and overreact to things, which is exactly what terrorist are looking for. Declaring a state of emergency is not an effective solution.