Isn’t keeping 7 with this better than mulling to 2 with this in pretty much all cases? I don’t think mulling, with the intent of casting this spell, gets you any closer to finding something with it. The top cards of the deck are unknown in all cases.
Depends what your deck is doing. Dredge for instance would always mull a 7 hand with this in it and no Bazaar I think, because they can take subsequent mulls after that. It's not until they hit like 3 that they have to weigh if this is better or not, and in most cases it will be because even if you whiff you get 5 cards closer to what you need where as with mulls you would not.
A deck that plays in a more traditional way may not need just one card though, it is going to depend on the combination of things they have in hand, what they know about the opponents deck. etc. There will also be causes where people keep hands with this solely because it is a green card to pitch to FOV.