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This was the official line when I worked for one of the Big Five a few years back. The rationale was that since SPs are tied to particular implementations (PL/SQL vs T/SQL vs ...), they unnecessarily limit technology choices.

Having lived through the migration of one large system from T/SQL to PL/SQL, I can understand the argument. I think it's bit of a canard though - how many places really move from one database to another on a whim?