My ideas about using nofollow attribute and paid links are: Google does not want webmasters manipulate PageRank. Simple & realistic way for expressing something that many people actually does not like and/or misunderstand.
So you just think this: you can send some useful traffic to another friend site, while you will not inflate PR.
It is very simple, i.e. if in dubt about an outbound link, use nofollow. Just my thoughts about this, of course. After all, nobody ever said that nofollow links are unuseful for ranking, just does not activate PR contribute, i.e. not entire link juice, if you have this problem.