The Wall Street Journal argued against it Wednesday.  I disagree ( a surprise, I know).

Nor is such a referendum permitted by international law….

This is a domestic Spanish affair; dragging international law into the matter is just cynical.

…they [Catalonians] fail to acknowledge the price all Spain pays for the national defense and diplomacy that keep Catalonia secure.

This is a cost that Spain no longer would have to bear if Catalonia succeeds in secession. As the Spanish, Catalonians, and Tunku Varadarajan, who wrote the piece at the link, well know.

The separatists also do not mention that they would have to assume around 20% of Spain’s national debt were they ever to secede, and surmount a Spanish veto to be readmitted to the EU.

The one isn’t demonstrated; it’s only a claim by Spain. The other assumes they would want to join the EU; this is not a foregone conclusion, either.

Those who oppose Catalan independence are not going to show up to vote on Sunday.

That would be their choice in a democracy.

The only real argument against the referendum is its illegality under the Spanish constitution and a top Spanish court’s ruling. But, then, so was our own gunpoint revolution against the Crown illegal at the time.