You can use cheaper, tapped creatures with powerful enters effects, and combine those with massive lategame bombs
On the back end, you wouldnt really want to play a four-mana enchantment that grants the ability to create a single 1/1 flyer each time the enchanted creature deals combat damage
Increasing the mana cost will make Teferi less effective at controlling the board, and the new static wont incidentally stop cards like Finale of Promise and Dreadhorde Arcanist from working.
Furthermore, simply telling players to ignore that until your next turn is much, much more intuitive than explaining the vagaries of flickering, and there are very few situations where phasing effects need any more thought than that
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You play it when you need to or when you have spare mana and it is always good (assuming you can afford the minor life cost)