I keep and carry a firearm for exactly the same reason I maintain my CPR certification and first aid training.
No matter what laws you pass, or how heavily you invest in medical (or protective) personnel, people you care about are at risk of having their heart stop (or having someone intent on hurting them) when those resources are minutes, or more, away. When that situation occurs and I am present, I don't want to live the rest of my life knowing I stood there and watched the person I love die because I chose, or was legally prevented, from preparing myself to deal with the situation. Whether preparation means getting training in first aid and life saving procedures, or carrying the best (easiest, most lethal, safest, etc.) weapon I can afford, in the hopes that whatever preparation I have done will be enough to keep the loved one alive (beat the bad guy), I know that in either situation I may be outclassed, but the better prepared and equipped I am the more likely it will be that I can resolve the situation in my best interest.
Personally, I have never had to use either my firearms or first aid training in a real world situation, and I hope never to need to, most who have either never do. But (Cool Story Bro) last week, my partner used her CPR training to save her boss's life. She did CPR for the ten minutes it took to get an ambulance there, and actually had a heartbeat back and her boss breathing by the time they did arrive. It helps that she is a registered nurse, but doing it on her boss rather than a patient was a novel experience. I have family that has used a concealed pistol to make a knife wielding robber at a convenience store back down and run away (no shots fired). I hope that in either of those situations, I would have been able to perform as well, with equally positive outcomes.
So to answer your question, I think a law abiding, well trained, person should be allowed to carry whatever they can that aids in protecting themselves or others around them. That means the weapon must be targetable and discriminate, and generally the scarier looking or reputation the better. A laser gun ala a Star Trek Phaser, would actually be almost ideal, with minimal collateral damage and multiple power settings, including non-lethal. A rocket gun would, generally, not be, since it is always going to create significant collateral damage and can only be used at a range where running away and evasion would be a more sensible option. The bad guys can and will get whatever they think will overpower anyone they are typically going to meet, legal or not. My .44 looks and sounds very intimidating to someone only carrying a .38 special, even though either one will kill a person just as dead. And either one will definitely send someone with only a knife running.
I hope that helps you understand. BTW, EVERYONE I personally know who carries a firearm (at least 9 that I know of) would give exactly the same reasoning for why they do so. For some reason though, you never hear that in the sound bites on TV or the radio. All I see get on TV are folks that *I* call gun nuts defending the 2nd Amendment on hunting or anti-government grounds, that, quite frankly, embarrasses me.