The danger, or maybe just annoyance, with #2 (with only accepting friend requests without intent to do more) is that they then use the friend connection to further advertise to your friends. I accepted one once - wasn't paying attention to mutual friend count or something - and was immediately spammed on my wall with some site ad and then they deleted their account. So some, rather than the slow scam game, are just playing the spam game.
Another danger is that with the extended access to your profile, they could get enough content to clone you convincingly, and then you get the scam where all your friends receive a duplicate friend request from "you", and the new profile tries to sell them on some scam.
Another danger is that with the extended access to your profile, they could get enough content to clone you convincingly, and then you get the scam where all your friends receive a duplicate friend request from "you", and the new profile tries to sell them on some scam.