[sorry if it seems again the ABC thing, it's not, or it is, but in a different way (I think)]
Focus is your productivity "power".
Focus depends on how sure you are of your choice (the level of trust in your guidance).
Many options (big lists) or continuous new inputs (unclosed lists) mean lower safety of choice*.
So i guess marking priorities = less options each time you assess lists = better focus = better productivity.
What I don't think is that you need 2 levels: YES or NO should suffice. The meaning is: "Do I want this on my next execution cycle or not?" (be it these next 3 hours or these next 3 days on my business trip). Each end of cycle, review all probably including new inputs (paused as possible during cycle). Reviewing requires reassessing higher level horizon guidance (which also should be prioritize bynarily, evenif most likely for a longer "execution" cycle).
So I would say YES for getting rid of items in front of you (marking priorities). NO for maintaining more than 2 levels because of maintenance/review/decision effort and time-volatility-trustworthiness of the 3rd level. If it's not for soon, get it out of your lists until next big review.
Goncalo Gil Mata
*(see very interesting Paradox of Choice - Barry Schwartz google talk)