On other hand, it's easier to carry a 3 pound 23 calibers thingy and scavenged ammo than the same ammo and 23 (x 3 pounds) pieces of different caliber spare guns...

But I agree with haertig, having these calibers quite neutered anyway, it could be more practical to carry a 12 ga smooth bore with a set of multiple caliber ammo holder inserts (It seems to me, I saw such a solution already marketing).

Personally, I would rather rely on reloading my .357 cartridges with any scavenged ammo components. The minimalistic pocket lee loader is under a pound + 1 pound .358x158 mold + a pound of primers should go for a long time. I understand the risks of using unknown propellant, but it is possible to figure how to ration it safely based on the scavenged cartridge size or, if time permitting, test firing varied loads. Also I've read that my Rugger GP100 is tough enough to handle load errors.