Its sort of the game designer's way of signaling, Hey, this card is important! Every new set of cards that comes out takes place in a different world (called planes), so the legends of each plane tend to pop up whenever a new set comes out in a plane the game has already visited
Directly drawing a card for only one mana also makes Bygone Bishops Investigating look quaint in comparison
Most decks run both if theyre aiming for that win condition, but Exsanguinate is positioned much better in the current metagame, where gaining life can often grant you another whole turn that you wouldnt have otherwise had access to
So not only does your opponent have to keep track of all the changes, but they have to be aware of that rules interaction
Opt is low value, it is the weakest card quality card in the cube
Ah yes, Mindslaver was printed in Mirrodin, which meant that Gleemax needed to have a new Oh no