Well, if you've hardy played it, why make that comment in the first place?
Leaning is vital to your survival, once the mercenaries, Monolith and higher grade Military start coming out geting shot in the body is usually fatal, regardless of the armour you're wearing. Being able to peak out gets to be rather handy.
It's a vital mechanic in SWAT too, also, Far Cry (1), Crysis, earlier Call of Duty games. It's handy for slowing down run'n'gun shooters, since barrelling along with (insert 'noob tube' name here) suddenly has an effective counter point.
It's a great mechanic, I miss it dearly from current shooters where ping and one-hit weapons make surviving encounters tricky. But then I miss persistent health and AI that does something else than swarm'n'duck too.