You donīt have to do differences training to fly it - any more than you have to do differences training to fly any other aeroplane in the "class".
Oh yes you do! Differences training is required between any and all types of MEP aeroplanes. If, for example, you complete the MEP rating on a Duchess, JAR-FCL 1 mandates differences training before you fly a PA34, and again before you fly a C310, or a DA42. The very reason that the UK CAA was unable, as it wished, to mandate specific differences training to/from the DA42 was because it was the opinion of the JAA that it was already mandated. Hence we ended up with the 'advisory' AIC.
Moggiee's robust defence of the DA42 is doubtless influenced by his own vested interests as one of the early customers.