A residence card, in most EU countries, is not considered a valid travel document to cross internal EU borders. A residence card, togeather with a valid passport, shows you are legally in the Schengen Area.
You should rid yourself of the notion that just because there are no regular border checks does not mean you can travel without required travel papers.
Spain has the strictest requirements in the EU: you must carry required documentation at all times (as apposed to others where you are required to only to have them).
Any check that shows that your passport has expired will lead to problems, since you don't even have a valid travel document.
Therefore allowing your passport to expire and then go to the country with the strictes rules is truly a very foolish thing to consider.