Because, to reiterate, if you have any sense of map awareness at all, you can predict these things without the need for tp. If 2 enemy players are suddenly pushing on your undefended tower, that is already a team fail. TP just lets you cover up a mistake you have already made. If you don't make the mistake, you will never get caught out.
It is not hard to visually scan the mini map to see who is miss, and where your team mates are, what the threat is (If your visible to the enemy, the enemy will be drawn to you). From this information, 9 times out of 10 you can predict with absolute certainty where the action will be, and where is safe to farm.
teleport gives you map control. having map awareness is good but theres a difference between knowing what might happen and being able to actually do something about it instantly...
i personally do not run teleport but i have seen it used very well.