I've watched some campaign scenario playthroughs and in the menu for advertising campaings, the description states that the ad campaigns boost the "daily returning visitor" numbers.
I guess 6000 per day is indeed the max amount of new guests and in combination with the returning (probably staying) guests you will get higher numbers. Depending on size and rating of your park and impact of running ad campaigns.
Just a guess, based on ingame descriptions from gameplay videos of course. But if this is actually the case, I'd say i might somewhat like the basic idea. If the limit is too low is of course a different topic and depends on how it plays out in practise in larger parks. We'll see that when the time comes.