Sorry I'm late to answer this, and maybe someone else has already done so. The point is to discourage self-voting for garbage posts. If a post is worth something, of course it should be self-voted.
But if the post is garbage, the philosophy is to make sure that if the author is the only voter that he will have wasted his vote. Thus, you want the author reward curve to be steeper on the right than on the left. The way it's done in the current implementation is that at the far left (when the post hasn't received any votes yet), the curve is a straight upward-sloping line; after about 400 Mvests are voting for it, the curve starts to bend upwards.
The proposal in this post is to make the curve start out completely flat and gradually bend upwards until it's a straight line. Both discourage self-voting in the same way: they make sure that a post needs a little "oomph" to get off the ground, so to speak.
A post with little weight on it doesn't make it garbage.
The calculation is based on voting weight , it doesn't calculate how many vote were on a post, it calculates the weight.
Someone who want to upvote himself is going to upvote regardless of any oomph. This feature doesn't discourage selfvoting at all, it actually encourages people to buy more steem power to self vote themselves.