How did you feel about Season 9?

[Kirayuki]

I think in answer to the question you posed, it would be Lucas' suitability that got him into the service. John was not suitable.

This works out so much more simply if you look at the character split of Lucas and John in the light that I do.

When John decided to become Lucas North, he 'became' Lucas North. John Bateman failed to exist anymore. He only returned to existence later in series 9 and we all saw how un-suitable for the job he was. It is perfectly possible for the human mind to adapt in such a way that he would forget all about John Bateman. (Possibly from trauma of what he had done).
Someone such as Vaughn, who was a skilled manipulator before the whole identity crisis came about, would find it easy to become a trigger to remembering what the mind had shut off. Vaughn was also a key figure in the events that made John become Lucas. If he wasn't around in his life for over 15 years, then it would be like removing an important factor and bending your beliefs so that you forgot it existed altogether. The re-appearance of him would metaphorically smack you in the face and shout 'wake up!' in that case.

I know that's probably bending the question a little but i had to at least try to explain the way i view it before answering it fully. Sorry if it makes little sense.

So back to the suitability sub-plot and putting how i see it simply as i can to save space and time...

- Lucas was suitable for the job.
- The old (Uni student) John may have been suitable.
- New 'Lucas' was suitable for the job.
- New (post murder/post new 'Lucas') John was not.

If you want me to try and clarify or expand on my view more please feel free to poke me and i shall attempt to do so.