Two questions on delta-CRL

[Denis Pinkas <Denis.Pinkas@xxxxxxxx>]

In section 5.2.4 (Delta CRL Indicator), RFC 2459 states:

  The delta CRL indicator is a critical CRL extension that identifies a
  delta-CRL.  The use of delta-CRLs can significantly improve
  processing time for applications which store revocation information
  in a format other than the CRL structure.  This allows changes to be
  added to the local database while ignoring unchanged information that
  is already in the local database.

  When a delta-CRL is issued, the CAs MUST also issue a complete CRL.

  (...) Again, a delta-CRL MUST NOT be issued without a corresponding 
  complete CRL. 

The two questions are the following: 

1) What is the rational for mandating the issuance of a complete CRL each
time a delta-CRL is issued ?
2) Under which conditions could this requirement be relaxed ?

Denis