You should only log xctry time for flights made over 50 nm. All others, leave the xctry time blank. It makes you logging experience less complicated.
The 50 nm requirement only applies to certain airman certification requirements. Logging cross country between two points is valid for those seeking 135 privileges for their first job...and logging cross country for flights of 50 nm that don't include a landing at a point other than the point of departure is also valid for one seeking the ATP.
Many who arrive at those two critical points (applying for ATP, and seeking that first 135 job) may not have all the cross country they need to fill the boxes if restricting themselves to only logging 50 nm flights between two points. Keeping track of all the time in all it's relevant forms is important to someone just starting out.