You can have same direction or opposite direction traffic 1000' or 2000' below you. Going into an ED, you have a changing VS which is putting a lot on the TCAS system in a very short time as you start descending to start computing threat levels and suggested avoidance action. I would suggest a large plane would have trouble initiating the avoidance in the very short time available to it before you come blundering down through his level. You may well also be descending at some 7000fpm through the direct tracks of many other lower flying aircraft, and with INS systems, all will be on exactly the same line over the ground! Why not turn off that track and take most of the immediate problem away?