Perhaps it's just my eye's, but I can't see a difference between Helvetica Neue and Helvetica Neue Light.
I'd expect HN Light to be much thinner..
It is pretty subtle but they are different. Do you not see any differences for the same block of text? (You might need to run File > Restore Standard Fonts in the 'Font Book' app if you don't see any difference at all).
I've attached an example Hype document which should show what I'm seeing. The problem appears to be present in higher font sizes.
Font Test.hype.zip (19.6 KB)