Companies such as Gratis and OfferCentric (especially Gratis) are able to negotiate a much higher commission rate for their offers since they generate large volume. That is why they can make a lot of profit from requiring just 1 offer and a few referrals. Other sites that do the same usually require more referrals, or require higher-paying offers because they cannot generate the amount of volume that Gratis is capable of, and therefore do not receive as much pay.
Doing 3-10 offers by yourself means that the owner of the site will receive a guaranteed amount of money.
From my past experience, I think referrals are extremely difficult to accumulate sometimes; personally, if I were to do another free gift site, I would say I would much rather prefer doing a 3-10 offer site unless the referral amount is manageable.