Unless they are in your hand, in which case you will just cast one with the crazy mana you get from surge
While this isn't exactly a Dragon deck, the presence of four Niv-Mizzets plus Twinflame Tyrant can make up for the slightly inflated mana cost
In gameplay terms, its most similar to Venture into the Dungeon from Adventures in the Forgotten Realms : a scaling rewards track that (at least in Limited) certain cards will be able to advance with a one-off trigger
We've seen cards with similar abilities, like Lithoform Engine and Strionic Resonator, used as combo pieces in the past, and Weaver of Harmony probably has this potential as well, although being limited to only copying abilities from enchantments making things a bit more challenging
If you cast one, you won't have to pay an additional the first time you cast the other