First, the motion tracker in Halo has different settings.
1, no MT
2, only allies MT (stopping or crouching has no effect, as you always see allies)
3, everyone, MT (stopping or crouching removes a person from MT, unless firing or using AC)
4, everyone, no matter what conditions
Normal settings are #3, which have been in every Halo. It does not track all motion (ie crouching). Once you understand how to use MT offensively, you have the advantage a lot, an begin to notice how many people do not use their MT much. The people who complain about no MT are very much the people who learn to use it well, mainly campers or those who do not play competitively. It takes getting used to.
Part of Halo is having MT, and it was in many games before. Halo is balanced with a MT, but other games are not, (for example COD, as it would give away snipers and everyone else, it breaks balancing).