It's a keyword action, so it may indeed show up as an additional cost for casting a spell, but it may also be part of an enters effect, or the cost of an activated ability
Telim'Tor's Edict is nice in a steal and sac deck, etc
The only exception would be if multiple effects that altered a Creature type were in play at the same time, particularly if they're using replacement effects (i.e., changing a Creature's type from one to another)
In short, he represents the pinnacle of control and card advantage
In the aggressive decks it is so cheap and you are aiming to be ahead already that it is just a pretty good tempo tool
And both Commander and Oathbreaker are usually played with three or more players, giving each player a lot more information to keep track of