How can a pump evacuate all the fluid from a flat bottomed vessel like a sump pan? If the pipe has a flat, 90 degree cut, it will seal itself against the bottom of the vessel and you won't get any flow. If the pipe has an angled cut, you will get flow, but once the fluid level reaches the top of the angle, it will suck air.
Therefore, you will always get some residual fluid if you pump out your engine. However, you will still get some residue if you use the drain plug, just less of it. Whether it matters is a different question.