One of the biggest problems with keeping real estate during such a long down period is that the costs will deffinately rise in Spain either because of higher taxes or living in developments where you are fewer people that will share the costs because some people go bankrupt. It all comes down to personal circumstances though. If you are allready on the brink of bankruptcy why even bother? I would just hold onto my stuff since selling would not get me out of the situation. If this was not the case I would try and sell for basicly whatever price. This will not ofcourse be good for the rest of the economy but in such dire situations you look after yourself and your family.
As a private person I could care less if I owed the bank or tax authorities 500 000 euros or 50 million. I would still not be able to pay my debt off in the forseeable future.