Well, since I don't think the animation can be made to adapt to the environment, it might look awkward on occasion - i.e. you could have the rest hanging down from the front of the musket barrel when shooting from a window where realistically, you'd stabilize the musket on the window sill, or it might just disappear halfway into the ground while crouching...
But that's definitely no reason to not include such an iconic item/animation, even if it cannot reflect its use 100% accurately in all situations.