A system which can be abused will be abused...

The vast majority of posts by the majority of users have been, are now, and will be forever both ignored as well as mostly unpaid. And that's perfectly fine, as long as most account holders understand that basic principle of "popular" voting.

If the system allows self-voting and multiple accounts, why would anyone who can profit by doing so not do so?

Any system which depends on pure altruism is going to be manipulated by those with less altruistic motives.