Many reasons to avoid, and all boil down to violating the principles of the relational model.  Most obviously, the child tables are bags, not relations, and the lack of foreign keys violate the information principle, which requires all information to be in form of attribute values in tuples.

The biggest prectical consequences are loss of identity integrity in child tables (duplicates have a way of getting inserted when you would least expect) and loss of transparence in the model: you will sorely regret when you have to explain this to the next user of the model, and no, you can never be sure you will be the sole user for life.