There are about 5k active users and about 110k registered accounts... calculate a probable multi account use having these numbers in mind!

For the misuse issue:
Sure you can upvote yourself, but you need SP to do that.
Do some more math and there is nearly no possibility to get out the money you have to invest (to the community) in a way that makes the slightest economical sense.
Either you have to gamble with the steemprice OR you have to do A LOT of curation additionally to your selfvoting to get near a good interest rate.

BTW: dan has multiple accounts, berniesanders has multiple accounts, ned has multiple accounts. There are people with whole bot armies here. @anonsteem can even make you new accounts by paying him BTC (so there is no abuse in the free steem you get by creating an account)