It is true that SequenceIdentityGenerator works in places other than Oracle (any situation where you use a sequence and the driver supports getGeneratedKeys).  It is also true that combing them leads to a single JDBC call.

However you totally speculate about how many SQL statements that translates to and whether that helps or hurts performance.  Some databases and drivers implement getGeneratedKeys by issuing a second command against the database to retrieve the generated keys.  And even on Oracle, parsing and executing INSERTs with a RETURNING clause is more overhead than INSERTs without a RETURNING (sequence pooling is often far more performant than what you suggest).

But the thing to really watch out for with SequenceIdentityGenerator is the same 'gotcha' that all "post-insert generators" suffer from, namely the fact that the identifier cannot be know until the INSERT is performed.  Which means that that INSERT needs to happen as soon as the save() or persist() call happens (usually).  This leads to lots of insidious difficulties and problems.

At any rate, I definitely do not want this change upstream.