No character lock-in
Even though there's rarely more than a single Palatir in any given sideboard, Legacy is such an expensive format that Legacy players are willing to pay a lot for a fancy Palantir
I would expect that, had plot been introduced during the old block structure, this would be the only implementation of the mechanic in the first set and there wouldnt be all that much for us to chew on besides a little arithmetic
More often than not, one of the key pieces in an opponent's combo win is going to be an artifact or an enchantment
There is about a 1 in 64 chance of opening a nonfoil Special Guest in a play booster, and Aetherdrift introduced first place foil Special Guests into the 2-card box topper packs with about a 1 in 70 chance
Still, it counts for this list because it has renew sprinkled on it, but trust me, you'll be much happier recurring it from the graveyard rather than exiling it for some mere counters