Single Actions being included as Next Actions was a choice that was made way back in the public alpha days. There were arguments about it on both sides. If you want the rationale behind it you probably will want to search the forum from way back then.
AmberV, a couple of other ideas:
Would it be possible to collect your single action lists (SALs) in a separate folder structure? This makes a focused perspective more robust. For example, I have a perspective that focuses on just my non-work roles. I avoid having to redo this perspective by having a static set of top-level folders. By focusing on the non-work folders I get the results that I want. I don't ever have to update my perspective, because the folders at the top-level don't change. (Of course, the downside is that your SALs wouldn't “live” next to your other projects. That might be a deal breaker.)
More out there: would it work to temporarily change the style of your single actions? Before your review, you could change the style to white text on white background. After your review you could change the style back to blue on white (or whatever).
Be sure to use Help --> Send Feedback to submit a feature request that single action list items be filtered separately from regular next actions. Chatter on the forums makes me think that smart groups are coming of OF 2.0. That would be a logical time for Omni to make that filtering distinction, since we could then get the current behavior by filtering for “Next Action OR Single Action”.