Interesting question.
I think that most people, as in your case, link to open to a new tab if they are linking to another page and don't want a user to be taken too far away from the existing webpage. This applies for both external and also internal webpages. I find in a lot of cases though, external links particularly don't go to a new tab just because people simply forget to put in the code!
There is no existing research and evidence out there to suggest that opening a link in a new tab will have any value on SEO. The key question for me to is to ask why would Google (or any other search engine) attribute more/less value to a link that opens in a new tab? If this question can't be answered in a way that is sound and sensical, then it's safe to say that opening a link in a new tab has no effect on SEO.
For me there is no sound or sensical way to answer that question, and so along with no empirical evidence suggesting otherwise, its safe to say that a link opening in a new tab has no SEO effect.