I've been thinking about it, and really I think what a lot of the "problem" with balance at the moment is, is that there are no ranged enemies. Adding bandits with guns will DRASTICALLY change the balance of what weapons are good and add situations where you actually *need* to switch weapons.
Right now, the only range that matters is close range. The only enemy in the game that has a single ranged attack is the cop, and they still run to you. Not a single other enemy can damage you outside of melee range, so therefore, no other range matters. If you see a zombie a mile away, it's not a threat and it doesn't matter if you only have a shotgun, it will either come to you, or you ignore it. The shotgun is insane at close range and will churn the entire horde into paste, so you don't need any other weapon
Spears, shotguns, snipers, even the m60 etc will all be pretty heavily balanced against each other when there are ranged enemies. Being able to throw a spear currently is fun but not really that important, but throwing it at a guy with a gun who's shooting you will be a legit asset over the club