There's no doubt you are on the short end of the stick in a situation like the Paris attack, but being armed does several things.
1) It causes problems for the terrorists during the actions at the objective. Terrorists are in complete control during resourcing, recon, planning, etc. but once the shooting starts, people do crazy things. They may know about the bodyguard, or they may be strong enough that it doesn't matter, but armed resistance means there will be more risk to the terrorist at the objective. If the target doesn't justify the risk, they may move on to another target. Basically, having armed people results in a harder nut to crack. It's not uncrackable, just less likely to be attacked if they have limited resources. It also forces them to do the payoff calculation. Is it worth the risk/effort for a low payoff, or should they expend the same risk/effort on a higher payoff target. Either way, having armed people increases the chances they will go somewhere else.
2)Engaging the terrorists with force changes the tempo of their attack. Think of "speed bump". The terrorists now have to deal with someone else with a gun. They may kill you quickly, or they may need some time to do it. Depends on the situation, and frankly some luck, as to how fast it is. But changing the tempo of the attack will result in changing the entire attack. If they get into a gunfight with you, no matter how short, that buys time for others. More can escape, more time means more help may arrive. During the Cold War, the 11th ACR wasn't in the Fulda Gap to stop the Soviets. They were there to buy time. They were toast, and everyone knew it. Buying time was their mission. Sure, an armed citizen might be dead meat, but he might cost the terrorists enough time that others might live. He might still make it as well.
3)Fighting back is important in the bigger scheme of things. No one is under the illusion of what an airliner hijacking means these days. No one is going to just sit there passively and let it happen. Terrorists now have to forget the idea of taking over an airliner with a couple of box cutters. Once the mentality of fighting back is ingrained in the target, the terrorists have to factor it in at every target. That causes an evolutionary effect on terrorist attacks. How that evolves is up for grabs, but if it doesn't work they way they want, they have to take a different route. Fighting back causes that evolution.
I make no illusion about the chances of staying alive with my LC9 and two spare mags. I do think the situation improves if I have them compared to not.