I have a use case as follows:
- 1:many record:class relationship (e.g., 'colors a person likes')
- 2 records may have different numbers of classes 
- Class order is arbitrary
- Potential count of possible classes could be very large
- Want a metric to compare the set overlap between those two records (e.g., 
'red**green**blue' & 'blue**yellow**fuschia')

As for the default delimiter, that came from a much more specific use case (patent category codes, which can contain spaces or slashes). But the choice of delimiter is arbitrary.