I thought about that when I was typing but I couldn't think of a game where it'd actually be relevant. Yoshi's Island has those divebombing ball enemies, but they don't "jump towards you", they jump in the background and fall in the foreground, and so vertical positioning / doppler effect is really the only important stuff and insofar as you can do that, you'd do it with 2.1. YI also has the final boss fight that uses the Z-axis but again I can't really think of how I'd use it.
Can you think of any specifics games, levels, or enemies in 2d platforming that would actually materially benefit from 5.1?
(I'm not sassing you either, because like I said I thought about that when I was writing but concluded it was a non-case IMHO)