Although I like the idea very much, some of the existing database deployments using InnoDB will be affected badly. I know at least one ERP company with huge number of tables in its multi-platform database, no single table has pk, but each of them have unique indexes that consists of not-null composite columns that are chosen by the InnoDB as the implicit pk. Since innoDB keeps the table sorted according to the primary key, and also since the columns of the primary keys are added to the columns of the secondary indexes, we have marked one of the normally not null columns to accept null values, that way forcing the addition of the documented 6-byte invisible pk column. That saved us and InnoDB is now handling successfully that bad database design.
Therefore, I am against this idea.