After reading http://www.php.net/manual/en/mysqlinfo.concepts.buffering.php:
Following these characteristics buffered queries should be used in
cases where you expect only a limited result set or need to know the
amount of returned rows before reading all rows. Unbuffered mode
should be used when you expect larger results.
Seem that unbuffered query are usefull only on select, because update, insert and delete have no result set.
I don't see any problem using unbuffered update, insert and delete except this:
Unless the full result set was fetched from the server no further
queries can be sent over the same connection. Unbuffered queries can
also be referred to as "use result".
So, are last_insert_id and affected_rows a result? I don't belive it, because no mysql_fetch* is done to get these infos.
So, if you experience an improve of performances using unbuffered query, use it!