As a guy who routinely carries a variety of calibers (38 Special, 9mm, 357 Mag, 40 S&W, and 45 ACP), I do like the 45 ACP. As an avid shooter I don't limit myself to one caliber, brand, style, or type of handgun. I want to be proficient in them all! Plus it is just fun to have variety. Now to why 45 ACP...
The 45 ACP round doesn't depend upon a lot of velocity to punch through things. The mass of these big bullets does the work. When the 45 ACP bullet loses a little velocity it is still able to perform. Some of the smaller, higher velocity rounds are more dependent on that speed to open up and "perform". When they lose some velocity, their terminal performance drops.
When you compare calibers don't think about the diameter, but rather the circumference of the wound channel. Blood loss occurs all around the full circumference of the wound channel. A little expansion in diameter will quickly increase the circumference and therefore increase the blood loss and wounding effect. Based on that, an unexpanded FMJ round at a .452" diameter will cut a bigger wound than some of the smaller bullets which may have partially expanded or totally failed to expand due to a loss of impact velocity.
OK, now for the 45 ACP fanboy smarty pants replies: "Because shooting twice is just silly" and "Because they don't make the 1911 in 46"...
To the modern self defense bullet designs... I think the tide has risen in bullet performance, however not all calibers increased their terminal performance equally. The smaller calibers such as the 380 ACP, 9mm, and 38 Special have gained more than the grand old 45 ACP.
Edmo