i have an ipod touch 5 and it has the lens stick out like this too, i've been using it without a case for almost a year, dropped twice, one time was at about 2 feet, the other time i accidentally tossed it to the wall and it landed on its back and still working fine.
i'm pretty careful but i don't treat my devices like princesses, usually i lay the phone on a table by dropping it at a height of about 3 cm, i never lay it down "gently".
and like i said i've been using it like that and i do it every day and it's still fine. the camera looks fine and works fine.
why? it's not because i'm trying to advertise for apple, but because of the material, the lens is made of sapphire which is extremely hard, that's why
i think that's the same for the nexus 5 too,
so don't get a case if you don't want to, i personally don't like using case.
btw this is going to be my first phone, it's time that i need one