I'm fairly sure that it doesn't matter if you participate but rather if you are eligible to participate.
That's what I used to think. In fact I made a comment very similar to yours (I forget if it was on a blog or forum), and was corrected by another commenter. I went and read the regulation(s) in question and agreed with their interpretation.
I'm not sure it matters that much. The following would all have to be true just to even make it a consideration:
a) No employer match
b) Make more than cut-off (which is higher than the median AGI)
c) Save less than $5000 into retirement accounts
d) A reason to push the person off the fence, e.g. mediocre to bad 401(k) plan.