That approach is perfectly fine with linear movement - because the
linear calculations aren't affected significantly by how large dt is (or
put another way: x += vx*2 is nearly identical to x += vx*1, x += vx*1
on your computer)
However, with any nonlinear physics (like say, gravity's relationship to
position, or an accelerating object) or with discrete actions that
happen once per frame (like say the artificial intelligence for an enemy
that decides what to do once every frame), then your game behavior can
change quite significantly depending on what values of dt you get.