If there is a user who is engaged in the platform and adding value through what they do, and they upvote their own (good) content, there really isn't an issue with that. As a stakeholder they are using their influence to direct the rewards pool to the content that they feel is adding value to the platform. It is within the 'acceptable constraints' of what SP is supposed to be used for.

If user is using their SP to upvote their own content above what it is worth, then that is abuse (in my view). I realize that this is a difficult thing to objectively quantify (which is why this problem is so difficult). There are quite a few cases that are pretty clear/obvious though. IMO, these are the ones that we should be focusing on.