In theory you don't need to save more than just the txid, you should be able to look it up the rest as/when needed.  However I didn't code bitcoind, I don't know every event which could result in it repository changing.  As someone with a background in enterprise "database/backend stuff" (official title) that doesn't leave me with a happy feeling.  I don't like the idea of relying on data which a third party (bitcoind) has direct access to.  If I put the data in a repository and strictly control access then it makes for a more controlled environment and that makes me more confident that there will not be any unexpected changes.

It may not be necessary, I just have never traced every possible condition that might result in the bitcoind changing information in the wallet that I may be unaware of.