I agree with the principle of what you're saying. A striking example from Windows XP is that is that folder icons are at a jaunty angle at 32x32 and higher but flat on at 16x16.
But what if you're looking at 48x48 icon on a screen with triple the pixel density in each direction? Surely you'd still want an image with with the same amount of detail as you'd normally get in a 16x16 icon, even those few details are rendered with a high pixel count – basically you're running into the resolution limit of your eyeball. And this is a very common situation now because of course phones have a far higher pixel density than most traditional monitors.
So there is potentially benefit in vector icons even though, as you say, you still need two or more versions with different levels of detail.
But what if you're looking at 48x48 icon on a screen with triple the pixel density in each direction? Surely you'd still want an image with with the same amount of detail as you'd normally get in a 16x16 icon, even those few details are rendered with a high pixel count – basically you're running into the resolution limit of your eyeball. And this is a very common situation now because of course phones have a far higher pixel density than most traditional monitors.
So there is potentially benefit in vector icons even though, as you say, you still need two or more versions with different levels of detail.