Casual hikers with no experience of Leave No Trace (environmentally conscious outdoor ethics) are more likely to have an impact on one single place. That's the main reason for the ridge runners.
Through-hikers live out there for five to seven months and so individually they have a higher impact in a years' time but they also are more likely to at least become more conscious of having less impact.
The impacts are more a matter of logic. Do you need to be part of a group to be comfortable at a shelter? And is that hurting the solitary experience?