One of the best parts about third person mode is being able to see exactly how camouflaged you are when you lay down in a bush or against some grass. Of course it depends on what you're wearing and where you're laying, but that just makes first person mode a bit awkward. Plus, if you lay in grass, you get grass in your face, and can't see over it. Yikes. It makes moving through fields and open space riskier as you have less visibility, so try and stay away from long treks on foot.