I always get frustrated when someone defines Bodhisattvas as people who hold off on entering nirvana until they have saved every other being in the universe. It's my Buddhist pet peeve. I've heard it put a better way: Bodhisattvas don''t wait to reach enlightenment for the sake of other beings; instead, they attain enlightenment in order to help others. The first definition makes Bodhisattvas out to be doormen holding a cosmic door for the rest of us to pass through into nirvana, like it's a lobby or theme park. This view, in my mind, tends to make nirvana into a destination, the way other religions do heaven. It objectifies enlightenment, and I don't think that it fits well with the Buddha's teaching. Nirvana--whatever it is: the extinction of craving, the Unconditioned--isn't some thing you can hold off achieving, a button you can press at your leisure, or a line (que) that you wait and opt out of because you want others to walk ahead of you. "No thanks, I'll let everyone go ahead of me."
I'm sorry if those are lame analogies, but they're my best attempts to understand this perspective. I just don't understand the logic. Why would you wait to reach enlightenment? Wouldn't being enlightened be the best way to help others? It's the old sinking ship example: put on your life jacket before you try to save others, lest you both drown. In other words, you can't help others until you've straightened yourself out.
Just look at the Buddha himself. He was so influential and skilled as a teacher precisely because he was enlightened. Awakening isn't something we should defer--as if that's even an option--but rather the Way, salvation itself.
Photo borrowed from Creative Commons flickr user Casver/Rosemania.