It isnt a legitimate tactic because it requires absolutely no skill or map knowledge to do. The same people who take advantage of the crouch spamming are the ones who throw all of their grenades at one target and normally only use shotguns. They do it because they know it works. If it didnt make them immensely harder to hit, they wouldnt do it.
And flanking them or using other means to get them isnt usually an option because there are other skilled players on their team who will take me out because im focused on avoiding this one kid.
And they can also line up a shot more easily this way by simply ignoring what they see crouched. Since theres no motion to it, its just as easy to line up a shot whilst crouching repeatedly as it is while just standing still. The only difference is that they cant be hit as easily.
If you ever played halo CE on pc, its the equivalent of going outside the map on death island, where you cant be shot but you can still shoot other players