When I play a rebuy, I always plan to make the maximum rebuy at the beginning and also plan to add-on at the break. If I can't do that, I won't play the event. The reason for this is expectation.
Take an example where you get 1500 chips for every rebuy. Let's assume that two people both play and don't get a huge hand for the entire rebuy period, but manage to keep up with the blinds. (This is a worst-case scenario, though I have had it happen to me.)
The person who rebuys/adds-on as much as possible would start the post-rebuy round with around 4500 chips. If they can get a hand to double-up with after the break, they will have around 9K.
The person who doesn't rebuy at all will have 1500 chips, and will be at around 3K after a double.
The advantage for the person who rebuys increases by a factor of 3 for every time they double, even if the other person keeps pace and doubles at the same rate.
This is why people who sit down with a single buy-in almost never see the money in these events.