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The Razorback, a tribal shield the Sniper wears on his back, has several problems that puts it at a disadvantage. When there are several spies in the game, the Razorback becomes less important since it can only be used once, making the Sniper vulnerable from behind again.
The Razorback has no benefit against any other class except for the Spy. Yes, Spies like to hunt down Snipers, but it's inevitable other classes will come after them too. When Snipers face anyone else besides Spies, they could use the Jarate and the Sub-Machine Gun for help since both weapons can work against any class in the game.
Also keep in mind that a Spy with the Ambassador can just finish you off with two bullets to your head. This scenario occurs when a Spy can see you wearing the shield; if the Razorback had been invisible, then Spies would have a harder time dealing with Snipers.