My answer would be vote selling hurts if steemit wasn't a complicated platform. Steemit is a very complicated platform and the effect of vote selling is hard to calculate.
For example at first glance one would clearly go this is a bad idea. However, when you add into the scenario bad distribution, bad curation or none at all. You begin to see without both manual and automated systems steemit may actually be worse off without voting bots and the like.
Let's consider a steemit without vote buying and selling for a moment. Sounds good but you'd in theory have less distribution. For example, the largest holders of sp may also be the most busy people. Which means they may do very little in the curation department if anything at all.
Let's say to remedy that they delegate their sp to some other person who can go about curating. Well now you have alot of influence in power in the hands of one person with a specific opinion and set of ideas. This person may only be interested in a particular subject of steemit posts.
In short as bad as vote buying sounds. It may actually make the steemit economy more sustainable. That is a theory but based on the already bad distribution on steemit. Massive concentration of influence ot less than 1%. Then statistically its no reason to believe distribution would be better without an automated voting service. If anything it would have to still be worse.