It's all pretty much been said so far. For the average shooter where all they do is go to an indoor range once a month, then yea, I can see your logic for not owning more than one gun in the same caliber. For those like myself who enjoy multiple aspects and types of shooting, owning one gun/caliber is very limiting. For example, I shoot USPSA/SCSA. Using my G34 as a carry piece would be quite difficult and possibly dangerous. Likewise, using my carry G19 for competition would be quite a disadvantage.
I will also say that I absolutely do not like your line of thinking because I feel it can lead to a slippery slope:
Why do you need more than one gun/caliber? ---> Becomes illegal to own more than one gun/caliber.
Why do you need to own more than X calibers? ---> Becomes illegal to own more than X calibers.
Why do you need to own more than 1 caliber? ---> Becomes illegal to own more than one caliber/gun.
Why do you need to own anything more than one .22LR? ---> Becomes illegal to own anything more powerful than a .22LR.
.22LR isn't really effective at anything, so you don't really need to own it either. ---> All firearms illegal.