Teleport is mandatory on 6 people per game
Its not. Unless you are in a pro game, and even then its only good because the "close" fights happen much less often so there is less use for ignite/heal/barrier
boring, uninteractive summoner spell
Its not. As a tp user you have to be sure nobody can interrupt your tp and nobody can ambush you at target point. You also have to make sure you are not losing much on your own lane if you are tping to another one. It needs a bit more thought than "press a button to guarantee a kill"
bad players getting rewarded for dying and teleporting back
They do not. They still gave you 300 gold, they just don't lose as much minions. A heal/barrier/cleance could probably save that player so he will neither give you a kill gold nor lose minions, and ignite could possibly turn the fight around so that you would be the one dead instead, and all this for 2x less cd.