In part it may be why JB was apparently so angry during the filming of series two - not only was the shooting schedule wacked but there's no clear motivation for anyone's characters. *sigh* After all a Jack Harkness who is haunted by a lost brother AND two years worth of missing memories should be a different character to someone who is haunted by one or the other lack. And again with Toshiko imprisoned in UNIT's Belmarsh... Very different backstory to her being recruited in some other way.