Mine came off too, don't worry, no problem.
Honestly I used to not even keep my cover plate on.. found it too annoying to put back since the fit was so tight.. every time I tried, the digitiser would snap off and I'd have to keep reseating it.
I've left my plate off for the majority of my phone's use and the phone works fine.. so you can either just do the bottom two screws and let the plate have some movement, or just not use the plate at all.
Right now, I have the plate on, connected by the bottom two screws (had to do a earpiece replace), still works fine.
It'd need a real good knock for any of the ribbons to drop out of place.
But yes, just be careful, the screw holes have some critical logic board sections beneath them.. I guess that's how Apple catches you if you put in the wrong screw.
The particular piece you've lost / snapped off is certainly not needed.