Good point, Ari, although I don't think it's just about the scale.
For instance, I don't feel that I need to have more than one partner at all times. After opening up my relationship with Raga I was de facto mono for two years and it was fine. Yet if he hadn't been willing for the relationship to open, these two years, if they had existed (read, if I hadn't broken up right away) would have been torture.
To me it's not about having someone else, necessarily, but knowing that I'm allowed to fall in love again if it was to happen. I just can't trust someone who doesn't give me that freedom, and if you can't trust someone, you can't share your life with them.
Because I can't control my feelings, I would never be fine with someone who demands I control them. Even without being in love with anyone else, knowing that if I did, I'd have to hide it or be "the bad guy" would be enough to just ruin my relationship.
On the other hand, I believe I could happily live with a single partner forever and never any other if I just happened never to fall in love again.
So I don't know about the scale. On the one hand I'm as poly as can't me (won't accept a relationship where monogamy is expected) and on the other hand, it's not something that I crave or need (I can be fine with just one partner, or even zero for that matter).