Upvoting ONLY yourself is not good I think, but I do it to keep my comment in the top to make it more visible. By buying SP, what cost me real money and gave the Steemit platform the ability to pay content creators (without investors no value for Steem), I earned the privilege to do this. Further there are no big advantages of holding a lot of SP. Of course I will up vote the post where I made a comment in too.

Also there is an incentive to create a quality comment, because you can up vote yourself only a few times a day and if you use these for posting bullshit, nobody else will up vote you and the extra visibility will not give you anything. I remark on myself, that now I have the chance to rank high in the comments, I put way more effort in the comments I place.

I think the investors are the only real risk takers on Steemit and have to be rewarded too. The inflation of Steem is way to high for me to just invest in it, but the fact that SP gives me the advantage to make my comments / posts more visible, I see it as a good investment. Only curation will not give me the satisfaction I invested for.