I agree. There is a lot of evidence - criminological, psychological, and statistical - that shows it truly is. Common sense even says it is. What is an armed, uniformed guard in a bank other than an evident means of self-defense employed in an attempt to reduce crime?
There is one theory of crime known as the routine activities theory. It says that a crime occurs any time a motivated offender finds a tempting target with a lack of sufficient guardianship. The interesting part of the theory is that it recognizes that how well protected something must be varies based on how motivated the offender is and on how tempting the target is. If the target is tempting enough and the offender is motivated enough, no amount of protection will stop the crime. If the target is not as tempting and the offender is not very motivated, it may only take a simple lock to protect the target. A visible weapon is simply an indicator of the protection of that target.
If you are walking around with large amounts of cash sticking out of your pockets, comic style, a visible gun may not protect you as the target is much more tempting. If you are walking around with normal clothes and no signs of extreme wealth on you, just being alert to your surroundings with no visible weapons may be enough to protect you.
And I will also admit that having a visible gun may make you a more tempting target, especially if they see you are not paying attention to your surroundings or if you are so old or frail that the offender thinks he can take you before you can use the gun. The gun actually becomes part of the expected loot and makes for a more tempting target.
I tend to keep my gun legally concealed by using a vest to cover it. I carry in an OWB holster on my right hip, where I always carried in uniform on duty. I do this because open carry is forbidden at work and because it alarms too many people who aren't thinking. But the vest was designed for concealed carry (has magazine pouches on the inside for example) and I think most people who look at me will recognize it as armed if they have any knowledge of firearm carry at all. For me, this is the best compromise between open carry and polite society/work.