Well that might work for one enemies Line of Sight on you... but just because one guy can't see you, doesn't mean another enemy isn't able to see you from a completely different angle.. and once you've been spotted... you are usually going to have multiple eyes set on you.
There's been multiple times where I've tried to evade enemy fire by ducking away somewhere, where I thought I was safe only to get shot by another enemy who got me from an unexpected angle.