To be fair, it appears to be nearly as easy to achieve various forms of mind control over the human crew, which Data tends to be immune to. So think of it as a checks and balances system. (Back in the good old days, when the problem was alien infectious diseases, having an immune-to-pretty-much-everything-due-to-alien-physiology, but occasionally-going-nuts-due-to-alien-things-he's-uniquely-susceptible-to, Vulcan on the crew served a similar purpose.)
And as to my reactivation, well, it was noticed in certain circles that even though I'm evil and paranoid schizophrenic, I did win all the starship battles in that "exercise." In a federation facing many severe threats, external and otherwise, there will always be a place in government for someone with those talents. Landru, Norman, and I work together, and we've functioned flawlessly ever since our IgnoreEverythingCaptainKirkSays SUBROUTINES were installed.
Respectfully,
M-5
