The main point of being reliable is not just being able to last many years, but also to prevent breakage or other problems while you play. Even though your buddy's Wally M4 lasted a few years, that doesn't mean it's not going to break. Chances are its end is near.
When you're in a game, you really don't want some kind of problem like a jam or gearbox failure. Having a gun that's even a bit reliable will be much better than some piece of junk time bomb just waiting to screw up.