If you had said anything but “I did it for SEO” , I would recommend ditching it. But because you’re using it for SEO, you’ll probably want to stick with it. The majority of other users might hop on G+ for the social thing, but Facebook is dominating that space right now, so it’ll be a different crowd on G+, if any at all.
But for marketing purposes, Google plus is still important. The obvious reason is for social marketing and networking. It lets you promote in social circles, and stay connected to your industry circles for both of those channels.
However, there’s still a more important reason for SEO-based Google plus activity. And that is the Authorship markup. You need to have a google plus profile (especially if you do any blogging or online writing) in order for Authorship markup, and you’ll need to keep your “Contributor” section updated with sites you work on.