Those creatures are now attacking that player
While you opening a Tree of Redemption in your $10 pack does mean that someone, somewhere is happily opening a Jace, the Mind Sculptor in their pack, this doesn't really do much to soften the blow of the bad boxes and packs that will be fairly common with Masters 25
They also pretty much suck at protecting themselves, which may warrant Spellbane Centaur or Sylvan Safekeeper
Once a card with paradox is on the battlefield, the keyword activates anytime an instant or sorcery is cast from exile, the graveyard, or the command zone
At the very worst, they're a tiny upgrade to a Guildgate or equivalent tapland, and therefore have a home in certain budget Commander manabases
A magnetic holder is usually most worthwhile when the card feels like: Not every smaller-format card needs this level of treatment, but some clearly do