Given a search engine like Lucene and a set of XML documents which need to be fully preserved, what are the advantages and disadvantages of using the search engine as key value store for returning XML doucments given a unique primary key which each document contains?
Read Search Engine versus DBMS. IMO, your application falls in the DBMS realm, and will probably be best served by a key-value database, such as couchDB. This is because you take no advantage of textual operations such as tokenization, stemming etc.