At the time, UR was the highest rarity in the Pokmon TCG but has since been surpassed by SAR
It does put you down a card, but it is still an incredibly powerful way to protect whatever you are doing on your own turn, and the ability to hit opponents' commanders and large creatures even while tapped out is notable
Historic cards can be powerful, but a more legendary or historic build is just significantly weaker than focusing on artifacts
The question is whether it's better to use Solemnity in this role over Anafenza, Kin-Tree Spirit or Melira, Sylvok Outcast, and while the answer is generally no (mostly because Anafenza and Melira can be found with Chord of Calling and Collected Company while Solemnity cannot), it's worth keeping in mind because, as an enchantment, Solemnity is much harder to kill than a random Grizzly Bear
This is perfectly justifiable, as Iymrith looks to be a future staple in many control decks, as well as, other archetypes
Because of this, you might get bored playing with that amount of cards