It's not a big surprise that, inevitably people abuse a system when they can. A person with multiple accounts can invest a large amount of money in all of them and drain the system just by upvoting his posts.

this is correct, but the multiple accounts are irrelevant. There is no advantage, in terms of SP and payout for a vote, in having the SP split among many accounts.

Multiple accounts are necessary for mining, which is why you see some people with a ton of accounts.