Promotion in a Vexilatio

[Nathan Ross]

a lot of centurions were travelling regulary to Rome for promotion and came back with news from the city.

Which period was this? Might the centurions perhaps have been promoted into the Rome cohorts (as many were, after about the mid first century)? They would then return (albeit probably to a different legion) several years later as primus pilus, which would have given the frontier legions a link with affairs in the capital... or was the sense of the comment that the travel back and forth was speedier than that?