Just because a card is going to be widely available doesn't mean it can possibly be unbanned. This card is obviously busted in combodecks and having fast access to the tutor'd card a la Gush, cantrips, Bazaar and the likes will give decks like Oath, Cephalid Breakfast, Reanimator, Scapeshift, High Tide and so on powerful plays and adds more redundancy to these combo decks. And well, the more consistent combodecks are, the more unfun the format is going to be.
Even controldecks will profit from Imperial Seal. At the moment there are still cards out there that just no-brain a certain matchup: Controlmirrors revolve around True-Name Nemesis most of the time, Moons/Ruination/B2B win against most multicolored decks, I don't think it's a good idea to give decks even better capability to tutor up such a card, jam it and then sit back and relax. Mind that Imperial Seal is very mana-efficient and plays quite smoothly with Snapcaster Mage for instance, and with Imperial Seal and Mystical Tutor in the format, I think Miracle-spells can also be relatively unfair (mostly thinking of Bonfire and Entreat).
Even controldecks will profit from Imperial Seal. At the moment there are still cards out there that just no-brain a certain matchup: Controlmirrors revolve around True-Name Nemesis most of the time, Moons/Ruination/B2B win against most multicolored decks, I don't think it's a good idea to give decks even better capability to tutor up such a card, jam it and then sit back and relax. Mind that Imperial Seal is very mana-efficient and plays quite smoothly with Snapcaster Mage for instance, and with Imperial Seal and Mystical Tutor in the format, I think Miracle-spells can also be relatively unfair (mostly thinking of Bonfire and Entreat).