Good polish is essential to achieve good light return. If the outside of the diamond is not polished well, there will be less surface reflection and light from the diamond will also be affected.
Factors that affect polish are bearding, scratches, polish lines, nicks and pits. To these we add the condition of the girdle, facets and culet.
It is not necessary for the polish to be 'Excellent' since are can hardly see these tiny defects. This is especially true for diamonds of lower clarity because the internal defects far outweigh the minor external defects.
However, if you are considering a diamond with high clarity, (FL to VVS2), then polish is an important factor as it affects the 'perfection' of the diamond and hence its price.
Having said that, do not every buy a diamond with 'Poor' polish either because its brilliance will be affected.
Most reputable gem laboratories grade polish accurately. So, you can depend on the diamond reports to determine a diamond's polish.