Jim Phelps, Paris, Barney Collier, and all the rest of the MI agents were cyphers. For all we know about their personal lives, they might as well have been mindwiped “actives”, living someone else’s life for the purpose of their current mission (er, “engagement”, to use the Dollhouse term).
It’ll be interesting to see where Joss takes us with this premise. His concept, that Caroline’s original personality will start to re-emerge in bits and pieces, could lead to all kinds of interesting storylines. Whatever happens, I suspect “Echo” will wind up having more personality than any of the MI characters.
(And to jump to another show, “Dollhouse” has a bit of the “La Femme Nikita” vibe, as well. The character of Dollhouse’s Adelle seems very similar to Nikita’s Madeline.)