The hard thing to distinguish is if the shot was intentional or not.
Such as if the shooter knows the target is behind a barricade, the boxes on the upstairs balcony for example, and they shoot the wall behind them and it hits. That seems intentional. But in saying that, if a target ducks behind the barricade under fire, and the shooter misses the target anyway, it ricochets off anywhere and hits would that still count?
Im on the opinion that a ricochet counts based on 30ish degrees to the wall, as illustrated:

This is based off a realism manner, a bullet will embed into a wall at anymore than 30ish degrees. Anything less, does have the potential to bounce off.
I personally prefer aiming for a direct hit, its just that much more satisfying...
But under the current rules, I call my ricochet hits too, regardless. Its not my problem other people cant shoot. It only means I need to get under better cover.
However, if for instance in the corridor you mention, a grenade is lobbed and explodes, of course that would get the 5 people in the corridor by ricochet, but only directly hit the first person in line.
So does this mean only the first person in line is hit? or the whole team in the corridor?
As I agree that spamming the corridor walls is cheap tactics, but so is spamming anywhere, either in attempt for double ricochet or aiming for the small hole in the walls.
Finally, when does a ricochet stop being a ricochet?
If a shot is taken at a wall, bounces off to another wall, then rolls slowly at your foot, does that count as a hit? Its almost impossible to know its made contact with you unless you explicitly see it.
As you'd also know, a shot loses momentum power each time it makes contact with an item. So a ricochet may not always be felt by the target, making them inadvertently cheating. (This is also an issue of wearing too much padding).
In conclusion, I think it should be more encouraged to aim for direct hits, than instead of hoping for a ricochet hit.
Experienced players will always go for the direct hit. New players don't know any better, and most want to get out and be trigger happy, as thats what they're there to do.
I don't want disagree with you, but I also don't support binning the rule.
I think it should be up to the DS of the game to specify whether they want to use this rule or not.