In some cases, extending or shrinking the parallel edges of trapezoidal surfaces without using guide edges can produce unexpected results; this occurs  because the extension is performed normally to the edges being moved.  In the case of shortened edges this can result in sliver-shaped remainders, because the non-parallel sides do not get shortened, as shown below:

Using the use guide edges option allows the guide edges themselves to be shortened, thus avoiding this issue.

Using the same starting surface, the following results are obtained under these circumstances:

 

Guide Edges Off, Elongation:

 

Guide Edges On, Elongation:

 

Guide Edges Off, Shortening:

 

Guide Edges On, Shortening: