Garrett, thanks for the question. I hate to give a complicated answer, but the answer is, "It depends." Specifically, it depends what you and the other SEO mean when you talk about "SEO." The phrase means different things to different people. Let me explain.
If you are referring specifically to helping a website rank more highly in organic search results, then the number of Digg submission, StumbleUpon shares, Reddit links, Diigo saves, and more will not help a website directly. These are not direct ranking factors (one reason, among many, is that it would be too easy for anyone to spam Google's algorithm via these methods).
However, these methods are useful in general online marketing (which some say is to what "SEO" refers) in ways that can help to increase a site's organic prominence indirectly. Here's an example. Say that you use social-bookmarking sites to "spread the word" about a site and its content. People find out about the site and its content via the social-bookmarking sites and then decide to write about the site, share it on their own social networks, and link to it. Then, the mentions and links that result indirectly can eventually help the site in organic search via the natural, earned backlinks.
Here's the key. Social media and bookmarking sites are public relations and communications channels that can be used to reach a targeted audience (for any purpose). One use is to spread the word in the ways that I described above to increase awareness of a site and its content. The more you do this, the more that referral traffic and the resulting mentions and links will follow -- and the more that organic rankings will likely increase as an indirect result. (Of course, do not spam.)
So, I guess my answer is that social bookmarking will not help organic rankings directly. But it can and should be used in a strategic way in terms of what is called off-page SEO, public relations, or content marketing (depending on who you ask) to help long-term organic rankings, sales, conversions, and more over the long term indirectly.
I hope this answer is clear and makes sense!