That's because it locks the vision angle, not firing? You cant see outside of that narrow area which gets bonus vision. It also slows the tank down....
It's a bonus with incredibly clear and balanced trade-offs, if you werent so biased youd probably be aware of that
You are confusing 1v1 situation, let's say SU85 vs P4, in which the trade-off you talk about will help a p4 coming from the side. But in real life games this is almost never happening since soviets have those "balanced" satchels so going close to the SU85 is guaranteed p4 death. Even without the satchels, just a regular AT nade will stop the p4 giving time to the SU85 to rotate.
But the range increase it's ALWAYS useful !
In all my games against soviets i never saw an SU85 being hindered by that ability, but i felt the range increase every game. Even if you flank it , he can just disable the ability