In Linode's case, a referral system isn't necessary, I believe. The service is so good that word-of-mouth advertising probably brings in more sales than a referral system would, and in addition, for people like me the addition of a referral system would lower my opinion of the companies using them - and assuming enough people like me, that could cause another noticeable drop in sales for Linode.
So I really hope the answer to this one is no specific rewards - except what we all get, as Deckert pointed out. Linode do enough for us already.
I recommend them whenever someone talks about dedicated/virtual hosting
To me, that would be enough.