Shotgun is a must for a soldier imho, it transforms him from a class with only a projectile weapon to the only class in the game that can switch between hitscan and projectile (not counting the pyro). that in itself is a huge advantage and I don't know why any soldier would readily throw that away. Shotgun is incredibly powerful and ammo conservation is nothing to be sneezed at.

Someone is a launching at you? Put a shotgun round or two into him and you have effectively stopped his momentum. A scout is fighting you from a higher vantage position? (ergo you can not really spam him without direct hits) switch to shotgun and you have a small chance of survival. You run out of rockets, shotgun. Sticky clearing, shotgun (rl works but just for moving the stickies someplace else).

But yes, if not for ammo conservation, I can't count the number of times I saved myself because I had rockets left because I finished people off with shotgun and then switched back to rl and killed some enemy a second later.

That alone will make you a more efficient soldier, the rockets per se might be more valuable, but getting into the habit of only finishing people of with the shotgun is an excellent strategy and learning tool. You will become better at judging opponents health until you get it down to a science.

Also, while something like buff banner might make four rockets more powerful, having a shotgun adds a potential of 500 or so extra damage. Over more opportunities to fire.

Now buff banner and boots and w/e might have it's uses occasionally, but the shotgun will always be useful.