I am going to answer this thread by throwing in my ideas as they jump to my head. My writing might not be coherent in terms of listing an argumental discussion, but i will just narrate things that I experienced first hand and why do I keep the 40SW in my handgun collection.
When I started shooting in 2009, i started with 9mm. Pretty soon going into 2010 there was a huge ammo shortage and the ammo shelves were empty except for 40 SW ammo. And that's when I bought my first 40 SW handgun an HK USP 40. After that I got a glock 22 and a sig P229 in 40. So what initiated me to shoot 40SW was really the ammo shortage of 9mm during the panic times.
After I started shooting 40SW occasionally i realized that from a defensive point there is really not much advantage over 9mm. However, I started realizing that I am developing better techniques to control the 9mm handguns more effectively. I started a theory that I like to practice with 40SW on a handgun platform and shoot competitions with 9mm. I still do that from time to time. Shooting a 40SW handgun keeps you on your tip toes in terms of paying attention to the grip fundamentals controlling recoil and muzzle rise. I usually explain it to my friends it is like going to the gym to stay fit and in shape for playing other sports.. Shooting 40 keeps you on your top shape in managing recoil and muzzle flip for shooting competitively with 9mm.
As of 2010 I started reloading, what I realized is that 9mm round is unforgiving in terms of accuracy. the round needs to be loaded in close proximity to maximum range to maintain good accuracy. That is not the case with 45 ACP and 40SW.. These two rounds could be down powered and still maintain accuracy and surprisingly same impact point at certain load ranges. (plus or minus). Playing with a 40 SW round it could be loaded to power factors similar to the 9mm round (in proximity of 125,000 as per IDPA standard) and what is interesting is that the recoil will be buffed down to a 9mm recoil) with maintained accuracy. So if I run my 40 in an IDPA match I will not be at a disadvantage from the typical 40 recoil as compared to people shooting 9mm. Yes I know that it makes more sense to shoot 9 all the time, but there is always an option to shoot minor 40 loads and not be at a disadvantage. May be i was too lazy and didn't reload 9 for the match, or I am out of money and short on 9mm bullets, etc.. it is always nice to have a back up plan and know your options.
Another reason I keep the 40SW in my calibers is because I shoot bowling pins matches.. Certain bowling pins setups are friendly for 9mm (narrower shelves) but the norm is wider shelves and you need enough power factor to knock off the pins off the table.. 40 and 45 is what most shooters use in this category. I toggle between 40SW and 45 ACP when I shoot pins. Plus sometimes it is so much fun to shoot a hard recoiling gun and maintain a respectable pace of energy management, it is just pleasing to me personally. Something like racing a rough car and winning with it.
Do I need a 40SW, the answer is no. because I compete mostly in 9mm, I carry mostly a 9mm, and when power factor is needed in a bowling match a 45 ACP will do the job.
But is there a place for 40SW in my collection, the answer is Yes. Because it is readily available in periods of ammo shortages, very forgiving to reload from minor to major power factor, the recoil could be used for extra training in recoil management, and it is always good to have more options if your budget allows it.
Add to all of this the price of guns in 40SW.. They are at about 20 to 30% lower in the same platform and model as compared to 9mm.. If you are a reloader and like to have that particular model and want to save money, you can buy it in 40 SW and your options are: either down power your reloads to minor PF and enjoy the lower recoil, get a 40 to 9 conversion barrel down the line, or just shoot it in 40 and enhance your recoil management techniques to shoot it effectively to a 9mm handgun.