I recently became concerned about the level of confusion on this topic, and especially, the alleged requirement to "renew" the certificates issued after 5 years, the claim being that they "expire". These are the certificates that some still refer to as "residencia certificates" though that is not actually what they are. In consequence of this, I contacted the European Commission in Brussels for clarification. This is their response:
"The question you raise concerns the application of Directive 2004/38 in  Spain which is transposed under Spanish law by virtue of Real Decreto  240/07. 
Residence documentation: 
Your underlying  question concerns the kind of document which Spanish authorities issue  when an EU citizen (eg a UK citizen) requests confirmation of his right  of residence in Spain. 
In pursuance of removing administrative  burdens for EU citizens when requesting confirmation of the right of  residence under EU law, please note that Directive 2004/38 abolished  residence cards or permits for EU citizens. 
Accordingly, EU  States may no longer issue residence permits or cards to EU citizens,  following the entry into force of the Directive in question (2004/38),  on the 30th April 2006. 
Instead of a residence card or permit,  Member States may only demand that EU citizens register their residence  in the host Member State by way of a Registration Certificate (article 8  Directive 2004/38). 
The recent novelty in this respect concerns  the appearance and looks of the registration certificate, which has  been made to look like a card as opposed to the A4 sized certificates  which were issued previously. 
1.   Registration certificates which are issued to EU citizens under article 8 Directive 2004/38 do not have an expiry date. 
Registration certificates contain the following information: 
Name, Surname, Nationality, Address, ID number and date of Registration (Article 7 (1) Real Decreto 240/07). 
2.   Permanent  residence is obtained under Article 16 Directive 2004/38, upon having  resided legally for a continuous period of five years in Spain. This  right arises automatically. There is no legal obligation to obtain and process this by way of application. 
Applicants have the option of applying for the Permanent Residence certificate, but they are not obliged to do so. 
Where  such an application is submitted, the relevant authority in Spain will  issue the certificate of permanent residence in question. 
This certificate does not contain an expiry date (article 10 RD 240/07).  If your question is whether permanent residence can be lost, the answer  is provided under Article 16 paragraph 4, which provides that permanent  residence is lost following the absence from the Member State in  question for a period in excess of 2 consecutive years. Your letter does  not justify what circumstances applied for the people mentioned in your letter (who are advised they must renew their applications for a  Registration certificate). If the suggestion is that the Spanish  authorities are requiring EU citizens who have permanent residence to  renew applications for registration certificates, this would appear to  be incompatible with both the Directive and the Spanish transposition  measure (RD 240/07)."
This is pretty clear. Spain retains the right to require you to enter your details on the register if staying for more than three months. However, you do not have to keep on renewing this registration. Thousands of people are being put to the uneccessary expense and inconvenience of long journeys doing just that - and as is obvious from the above, this is simply not required despite claims to the contrary by various abogados and officials (who seem woefully unfamiliar with general EU diectives and their effect). I would also point out that "legal residence" in Spain is categorically not dependent upon any form of "registration". This right arises automatically (subject to some limited exceptions set out in the Directive and as defined in recent European Court of Justice decisions). 
This has wide implications for all EU citizens in Spain. Currently, many are being put to the trouble of "renewing" what is coloquially referred to as "residencia" when this is absolutely not required. One registration is sufficient. It does not need to be "renewed". 
ACH