>What is the purpose of this behavior?
I suspect it is there because with something like
x\left(\frac{x}{2}+1\right)
the spacing is excessive. This is a thing that happens with \left and \right, and even with 6(x-1), MathObjects will print using \left and \right.
If that is correct, maybe the badness from using `\!` outweighs the good that is achieved from cutting that excessive space.
You can see `\!` being introduced in pg/lib/Parser/BOP/multiply.pm, and in a different way in vim lib/Parser/Function.pm. I just glanced at these (after grepping for `\!`) and maybe the former is the place to cut it, if that is overall the right thing to do.