I would probably put convenience at the bottom of the value proposition. For most of our users who are serious about growing their website, and not just posting to get easy rewards, the engagement from Steem to their blog is a big value. So with our plugin, you get to have all of the engagement (upvotes and comments) that you get through Steem, also appear on your blog in the embedded comment section. So if you're starting out, and hoping to show the world that you get some decent comments and engagement, using SteemPress and thus connecting your blog to Steem grants you an instant access to a community which in turn helps you grow your site when visitors have conversations to engage with.
At least I know myself that if I were to resume my space and science writing that I did before starting SteemPress on a new blog, I would be a lot more happy and proud to share those posts when they already show a lot of interactions and 10+ comments, rather than being all empty and sad :p.
That being said, it's true that for most creators who make below 10-15$ worth of upvotes, our percent of the vote is usually greater than the 15%, and thus making it more rewarding to post through us than not. (Of course this requires the content to be orginal and good, the blog to be serious, and the engagement to be decent).