MyDads38,
There are two operative 4-letter words when it comes to gun ownership, "Need" and Want". Depending on what use you intend to put a firearm determines the "need" aspect. If all you are going to do is plink cans and rocks then all you will ever "need" is a .22 rimfire, a pellet gun or a slingshot (AKA "Rubber Flipper").
What I think you are really asking is "What is the justification for the .40 S&W?" Based on "need" when placed between the 9mm & .45 ACP, absolutely none! But then the same can be said for any caliber. New cartridges do not make others obsolete! It's like having a .30-06 and asking do I "need" a (.30-30, .30-40, .308, .300 Savage.......)? Same answer, NO! The .30-06 will do everything the others will and then some. Everything changes when we get to "WANT"!
Do you "need" a .40? NO! Do you "WANT" a .40? Then buy one, if not, then don't. Just that simple. Whatever you do, DO NOT buy one because someone else likes the caliber!
FWIW, I ended up with several 9mm pistols, a caliber I never had an interest in, simply because I picked up several thousand fired cases that were simply left behind. Did I "need" a 9mm, no. Did I really want one, no! But I did have LOTS of good brass that I couldn't see go to waste!
There are two kinds of people in the shooting sports, "Shooters", and simply those who shoot! If you are one of the last you won't understand. If one of the first you already do.