my argument is readibility: when you specify a dict, then there are many ":" which (imho) decreases readibility, compare
'mock:type': 'mock.type:seq',
'mock.seq:id': 'a1',
'mock.seq:string': 'ACTGA',
'mock:description': 'some foo from bar',
with
'mock/type': 'mock.type:seq',
'mock.seq/id': 'a1',
'mock.seq/string': 'ACTGA',
'mock/description': 'some foo from bar',