Makes sense then.
As far as DC tags go -- custom columns are quite nonstandard as you saw, they need to define both content and parsing rules in the OPF. Using meta tags is I suspect deliberate.
Perhaps an identifiers type builtin column for storing DC metadata can be created. Theoretically. Kovid would have to decide it is worth it (and change always needs consideration).
What can you do with DC tags that you cannot do currently?