MGS4 is the only game I've actually played/beaten (I watched my brother play 1 and 2, and Chip Cheezum has done a great LP of the games). I always tried to do things stealthily as I could. After all, you're one man against an army. The benefits of nonlethal are that if someone discovers the body they don't go "oh crap someone's killing our mans" instead it's "wake up you lazy piece of shit". However, it was great fun using lethal weapons, especially machineguns where Snake lets out a hilarious war cry if you fired for long enough. Plus the OICW (or whatever it's called) that has the air burst grenades, because why not?
I just played Ground Zeroes yesterday and realized just how bad I am at MGS. It took around 80 minutes of playtime and I only got a B rank; had to reload checkpoint because the damn APC came around a corner and killed Paz in one shot. I know I'll likely do equally terrible in MGS5: I'll do at least some portion of the mission stealth/nonlethal, but I'm bound to be spotted at some point and then I'll have no choice but to go loud. Sort of like Payday (2) actually...
By the way, dropping a body and picking up a gun is the same button command... which is a bit infuriating. I want to put the guy down so I can shoot people, but instead I keep dropping the gun with a suppressor without realizing it.