Yea I posted a thread a few weeks ago about this same thing when I saw that I was missing this piece. My buddy who has a body shop brought it to my attention when he noticed stress fractures from the wind, since it was so flimsy without it, in the paint on my bumper/lip. This plastic piece would also give the lip support from the wind at higher speeds. We ended up cutting some thing metal strips and bolting them on to give it some support. Its worked so far but we kinda look at it as a temporary fix until we can get a new piece. The Nismo piece was like 1000$ or something like that when the regular 370Z piece was 200$
Check out this thread for more information on some of the comments:
Missing under piece to bumper/front lip?