You can see Google's problem here. They want the best SERPS... or at least they want the best balance of SERPS and earnings... yet they are aware that because links count in the algo, SERPS can and are manipulated.
Google's biggest issue is probably cultural. Niches like pron, gambling, pills and maybe even dating are lucrative largely because social and political velues drive them below the waterline and your average joe schmo probably won't want to naturally link to sites in these niches because they want to keep their social habits private. Hence I would imagine the vast majority of links in these niches are exchanged, paid for or in some way obtained "unnaturally".
Resultingly, it not only makes sense that Google treats different verticals in different ways but that these niches are where most of the spam problems are going to crop up. The problem is that regardless of the social implications, some of these sites are genuiniely useful to a large groups of people.
I would have thought that any changes to linking would (primarily) approach these sectors and aim at trying to identify where linking is unnatural. How, I have no idea how - that's the challenge - but I wouldn't be surprised to see any changes to the algos affect the impact of links, in these niches particularly.