This answer is based on my intuition, so it is to be taken cum grano salis.

The seeing is stated as a fact from the point of view of an action in the present, or so it seems to me. (I have never seen LLPSI!) This relation to present is a reason to prefer perfect over imperfect in my opinion. Using the imperfect often describes circumstances of the past ("I used to see him every day" as opposed to "I saw him every day").

Also the act being completed makes a good case for the perfect tense. It was repeated, yes, but repetition typically requires imperfect when the point of view is in the past. The perfect gives an impression of change, as the old action (described by the second vidi) is over: the slave was often seen here, but the last time I saw him someplace else.

This answer is based on my intuition, so it is to be taken cum grano salis.

The seeing is stated as a fact from the point of view of an action in the present, or so it seems to me. (I have never seen LLPSI!) This relation to present is a reason to prefer perfect over imperfect in my opinion. Using the imperfect often describes circumstances of the past ("I used to see him every day" as opposed to "I saw him every day").

Also the act being completed makes a good case for the perfect tense. It was repeated, yes, but repetition typically requires imperfect when the point of view is in the past. The perfect gives an impression of change, as the old action (described by the second vidi) is over: the slave was often seen here, but the last time I saw him someplace else.

The repetition is a good reason to pick the imperfect. The two tenses emphasize different sides and both are possible. I would consider this particular tense choice to be a matter of nuance. I would have probably chosen the imperfect, but my point was to argue that there are reasons to do otherwise.