A decorative element can be fine in a design model, but 1) a good design tends to have no purely decorative elements, and 2) it becomes problematic when the decorative element looks like a meaningful element but does not actually carry meaning (or the intended meaning).
We all recognise an icon in a menu as a meaningful element. Treating it as a decorative element is wrong and adds mental overhead, as we tend to scan every one of those icons (putting it at the beginning of menu text, i.e., to the left for LTR languages, makes it worse). It is well-known we do tend to scan these icons because that is the reason icons work: repeated exposure creates intuition. If this intuition is not put to use, then all such icons are a waste of our attention.
For example, a bullet in a list: fine (differentiates where each list item starts), window shadow or the 3D effect on window close buttons: fine (meaningful in terms if differentiating areas in the GUI, not pretending to do more); whitespace to set apart one thing more from another thing than from the third thing: fine (if that reflects the relationship between those things).
We all recognise an icon in a menu as a meaningful element. Treating it as a decorative element is wrong and adds mental overhead, as we tend to scan every one of those icons (putting it at the beginning of menu text, i.e., to the left for LTR languages, makes it worse). It is well-known we do tend to scan these icons because that is the reason icons work: repeated exposure creates intuition. If this intuition is not put to use, then all such icons are a waste of our attention.
For example, a bullet in a list: fine (differentiates where each list item starts), window shadow or the 3D effect on window close buttons: fine (meaningful in terms if differentiating areas in the GUI, not pretending to do more); whitespace to set apart one thing more from another thing than from the third thing: fine (if that reflects the relationship between those things).
This is all somewhat simplified.